I think you may have missed their point.

>for most people animations are not smooth

I don't know about most, but it's definitely true that there are processing limits inherent to user's hardware that make certain animations run very clunky, thereby turning a perfectly good animation into an 'intrusive animation'. Because we often have no control over the user's hardware we have 2 options, stay conservative and optimize things that improve the UX for everyone or ignore the possibility that certain users will have a bad experience and push forward with such animations.

Unfortunately, many product owners tend to build for only the 'happy path' scenario and choose the latter.