It is false claim but there is some wisdom to use inside and outside to make pocketing ball easier. Of course now we don´t talk about positional needs but only making target bigger.
Every shot have a margin of freedom on contact point where it still goes. If ball is close to pocket margin gets bigger and so on...
Inside English can make margin of freedom bigger .. IF ball is thinner than something like 35 degrees. If ball have very slight angle outside english will make margin of freedom bigger.
I am not sure (can´t remember) what degree is exact number where inside should be used to help making a ball if that is only goal. Maybe it was 45 degrees...
Anyways. Check Jack E. Koehler - Science of Pocket Billiards for details. I think that rail shot section was most entertaining read about physics in pool. It will even improve your game if you can bring that knowledge to practice table and then shoot those shots.
P.s that book is awesome so many ways anyway. I recommend to all who is serious to getting better to at least read it thru.