Calibre is 13 years old and widely used. There is no reason someone would want to not use it because of as trivial a reason as Python 2 to 3 except for some rationalized emotional nonsense about the Python 2 to 3 shift. Every time Python 2 vs 3 is discussed, there are a bunch of emotional people spewing vitriol about luddites or some nonsense like that. Easy for especially weakly socialized young people that many programmers are to get caught up in that.
It's especially hilarious because he is comparing projects done by bootcamp graduates to projects done by highly trained PhDs. Some people just don't have a clue about the massive difference involved in webdev type projects and complex engineering projects.
That is just the mathematical description of gravity according to general relativity. We don't have a theory for how the uneven curvature of space happens and we don't have a quantum mechanical theory for gravity either (gravitons are hypothetical, not proven).