Using a language suitable for rocket-science level computer science doesn't mean you are forced to do rocket-science level stuff, though. (I sure don't!) The barrier to entry is pretty much typing "OCaml tutorial" into your browser and then finishing the hello world program about 60 seconds later. Although the procrastination power of HN is perhaps a more formidable barrier to entry. ;-)
"Preventing abuse" is generally equivalent to surveillance - identifying a session as being associated with some longer lived identities. Their explanation makes me less comfortable for why they're doing it!
Redirecting to the https site is a bit less secure than the hidden service, in that your traffic will only go over 3 hops (TOR client) rather than 6 (TOR client + hidden service circuit). But AFAIK they could also write a modified TOR client that served a hidden service without any additional hops. Those additional hops are best seen as security for the hidden service rather than the user. Also, a hidden service for a well known business is kind of a pointless marketing gimmick.
If you are attempting to leak state secrets (as was the case of Edward Snowden) or going up against a powerful state adversary, email may not be the most secure medium for communications. The Internet is generally not anonymous, and if you are breaking Swiss law, a law-abiding company such as ProtonMail can be legally compelled to log your IP address. A powerful state adversary will also be better positioned to launch one of the attacks described above against you, which may negate the privacy protection provided by ProtonMail. While we can offer more protection and security, we cannot guarantee your safety against a powerful adversary."
Another huge plus for source based package managers not elaborated in this article is how easy transformations are [1], so you can rebuild your software with different compilers, dependency versions, source branches, etc. from often a single command.
I love this kind of stuff, but the barrier to entry just seems so huge.
https://ocaml.org/learn/success.html#FFTW