Linux and F-Droid are neat and all but are almost zero users.
Linux makes up 3% of the desktop market share. You could count ChromeOS as another 3% but that's pointless when using proton. You might as well use Gmail then.
F-Droid doesn't provide any stats at all but almost no one even knows about F-Droid unless you are a massive privacy nerd. Even then at least those I talk to still use Google Play while understanding where their information is going rather than abandoning it entirely.
Proton is trying to be a mainstream solution to encrypted and privacy-focused email. Getting it on Windows, Mac, Android (Google Play), and iOS is far more important to them than getting it on F-Droid, the popular Linux repos, or anything else. For good reason, in my opinion, they are making a business.
Privacy focused people use more often Linux so the percentage of Linux users for proton services might be a bit higher. I don't care about a calendar or mail app but I think a proton drive client for Linux would be important.
I question the validity of that. We don't really have any stats on that and while at face value, logically it makes sense but a quick search of that exact phrase gives me recommendations for all 3 major desktop operating systems. Cyber Security Experts I would say are a great representative of privacy-focused people and here is a bunch of them saying they wouldn't use Linux as the host OS: https://www.quora.com/Do-cyber-security-professionals-prefer-to-use-Mac-or-Linux-as-a-main-operating-system
While anecdotal and thus can't be confirmed one way or another that privacy-focused people use Linux, it creates enough of a question to outright dismiss taking the statement as pure fact just from the common sense argument.
Lastly, one thing we can do is extrapolate data. ProtonMail is a privacy-focused company with 70 million users. https://www.wired.com/story/proton-mail-calendar-drive-vpn/ So then if ProtonMail is privacy-focused and has 70 million users (far more than the amount of Linux users on Steam as a benchmark.) then why aren't they putting out Linux desktop support or releasing their app on F-Droid. One could extrapolate that this company which is privacy-focused and has 70 million users still doesn't see these things as important because they feel like they couldn't get enough users from supporting these things.
Potentially though, it might be bad to assume Proton is acting towards its own userbase and maybe for some reason it's actively acting against it. I feel like in that case, if I was a daily Linux user, I wouldn't want to use proton.
The fact of running an OS and other software that spies on you is proof against being 'privacy focused'. And many cybersecurity professionals use Windows at home, have dozens of devices with always-on microphones all throughout their house, use a host of cloud-based home automation, etc. It's just not true that working in cybersecurity means you do much to preserve your privacy.
And in practice today, privacy and security are in tension when it comes to desktop OS choice. macOS has a more destructive security model than most Linux distros, better suited to running proprietary software from untrusted sources. But compared to *BSD along with many Linux distros, macOS is also absolutely teeming with telemetry and cloud-centric functionality. In a word, macOS is more secure but less private. That many cybersecurity professionals would take that tradeoffs doesn't at all show that macOS has better privacy than Linux.
I feel like you are then gate-keeping the term privacy-focused to mean "You must use Linux otherwise you aren't privacy-focused!" Either way, this points back to my original point. You need data to make this claim that more privacy-focused people use Linux. Otherwise, the opposite claim holds just as much water.