Edit: At least it is if both the sender and recripient uses protonmail. Its open source so you could verify the client to make sure that the contents are encrypted. As for IP address and subject lines tho, that's up to the Protonmail company to honor your privacy, and just hope they don't betray you.
Protonmail is definitely more private than google or Microsoft, but you shouldn't hold 100% trust in any provider. Ultimately your data is still on their hardware and they have control of it. Also, as others have pointed out, both sides need to be secure otherwise all that data is accessible on the other side.
You can mitigate it yourself a bit by hosting your own email server, but I highly recommend against that as its a massive headache to secure and basically every provider will reject your messages anyway.
besides the above, their open support for regime change in China is NOT something a Taiwanese company would do. I live in Taiwan and have worked here for over a decade. Executives here try to keep their head down and just make money. They do not champion any causes.
I have read that blog entry and some of its references. The evidence provided for this strong claim seems to be very weak.
I would not judge anything based on the listed talking points.
Now, knowthing is impossible and such services are sure in the interest of governments around the world. I also want to remind people on the Swiss Crypto AG which sold compromised analog encryption machines for decades.
Even if you have 100% confidence in your own provider, you also need 100% confidence in every other recipients provider, which is basically impossible.
Of course there are alternatives. That's the cherry on top of this crap pile: only regular non tech folks are affected. Nerds and actual criminals will just run an xmpp or simplex server and not care about the legislation.