I suppose that is a way that you can think about it if it helps you to judge your stroke correctly. In my mind though, I see the two effects as having two completely different and (in my opinion) unrelated causes. First off, I will say that (again, in my opinion,) anything that Barry Stark says, believe it. Incredibly intelligent snooker guy and I think his presentation is excellent to make the point clear. Coaching is about communication....a coach may know everything there is to know about a subject but if he cannot effectively communicate that knowledge to a student, then he is not a very good coach. Just because someone is a very good snooker player does not necessarily mean that person will be a good snooker coach. Barry Stark is a fantastic coach, the whole package.
But more to your point, what I was trying to say early on about "straightening up", I believe it occurs because a firm strike of a ball INTO the cushion causes the rubber to compress, and then of course it de-compresses, pushing the ball away from it like it is a trampoline. However, the ball will reflect at a straighter angle because the rubber absorbs a lot of energy into it before pushing the ball away. This is where language fails me...I am having a hard time communicating this concept properly. I think BadSnookerPlayer hit this nail on the head when he described it as, "I think it therefore occurs because the deformation of the cushion is not symmetrical as it is deformed more on the ‘far side’ of the shot."
But the "sliding" is an easy concept for me to explain. Similar to "squeeze", "slide" is just a term that we use, a word to connect the idea with the reality of what actually happens in the physical world. (By this, I mean we use the term "squeeze" but nothing is actually getting squeezed at all and we use the term "slide" but nothing is actually sliding at all......these are simply words that we use so that others know what effect we are talking about when we say these words.) So what is it then that is actually happening when the cue ball is "sliding" off the rail? Easy to understand....
First, imagine setting up White on Green spot, Brown and Brown spot and simply strike Brown along the Baulk line straight into the cushion. Of course, it rebounds straight back at you. Now, same thing but take Brown away and strike White directly into the cushion. You can do one of three things: centerball strike White rebounds straight back at you, left side on White and it reflects toward Blue spot, right side on White it reflects toward Baulk cushion. So spins have an effect on how a ball comes off cushion. That is obvious. But what is NOT obvious is that the cushion will INDUCE spin onto a ball simply by the friction between the two surfaces.
Now, let's apply that knowledge you probably already had to this concept of "slide"....a similar scenario to Barry Stark's but a slight change. Black on Black spot and place White on a theoretical spot which is the exact mirror image of the Black spot on the Baulk end of the table and we are going to strike side cushion on Yellow spot side. Again, with the "equal angles" theory, you would need to hit the White to the space that is occupied by the middle pocket in order to hit Black. But you know that is not true. If you hit a spot on cush a few inches to Baulk side of middle pocket, White will "slide" off the cushion and strike the Black on spot. What happens is easy to understand....you strike White centerball, but when White strikes a GLANCING BLOW on the cushion, the cloth of the cushion has a gripping effect on the surface of the cue ball because of simple friction (very much like a well chalked tip has a gripping effect on the cue ball), so this gripping effect will INDUCE spin on the White. Might be easiest to think of it this way.....White WANTS to travel in a perfectly straight line. But the cushion gets in the way. ONLY think of the right hand edge and the left hand edge of the cue ball (again, assume like Barry Stark's video we are hitting the cushion on the Yellow ball side). When White contacts cushion, the RIGHT edge of the cue ball slows down dramatically through contact with cushion while the LEFT edge of the cue ball wants to continue travel in a straight line. This then describes exactly HOW the spin is imparted by the cushion. It is exactly the same as if you yourself applied the left spin with your cue tip. So as the White rebounds off the cushion, it "slides" off at a more shallow angle exactly like when you struck White along the Baulk line with left had side applied. Simple.
The more shallow the angle, the more glancing the blow, the more spin will be imparted onto the White by the cushion. This is strictly a matter of experience and judgement by the player. The truth is, very, very few players probably understand the physics of what is happening as I explained it above. Instead, it is just a matter of experience, and trusting the gut instinct, and learning over time, and simply referring to the idea as "slide".
If you actually understand what is happening and why, I believe you will dramatically shorten your learning curve and you will begin to play these types of shot much more accurately immediately. Good luck!