The video in question discusses all that, the primary point is that gross overestimates can actually be negative for system performance and lifetimes, that there is actually a right sized system for a house that can be calculated (with some overhead to be sure) and that installers are probably just being lazy and speccing units that are way oversized for the homes.
To make a car analogy it's like putting a huge engine in a tiny framed car. You can't actually get the performance out of it because nothing else is designed to handle it and running a performance engine constantly at 20% of its potential is actually bad for it.
2-5 C increase in temps is a bad thing for climate but the extra heat load on an AC system isn't actually all that much, maybe a few thousand more BTU/hr. Like for a 12x12x9 room you might need 12000BTU for a 15C differential but a 20C differential only adds around 2000BTU needed, that's 1/6 of a ton of extra AC
If you have the time and you are interested it's worth a watch, they are very thorough (that's why its over an hour long) and there is even a follow-up addressing common questions.