There might really be nothing to it, as the Proton* people claim. Or at least, just a somewhat iffy roll-out of their VPN, using Tesonet staff.
But on the other hand, I gather that Mozilla has picked ProtonVPN for its integrated VPN testing. And they seem competent and privacy-friendly.
Also, whatever they did with ProtonVPN, there's no reason to believe that there's anything wrong with ProtonMail. That's arguably their core competency. And they arguably brought in Tesonet because VPNs were not part of their core competency.
If you are migrating away from a centralized email provider you should move to a personal domain. That way you won't be trapped by any service in the future. There are numerous companies[0] that you can point your domain to and they will handle everything else. This costs a bit of money but it means that you will be the customer rather than the product.
[0] I use migadu.com but fastmail.com seems to be very popular with the HN crowd.
But on the other hand, I gather that Mozilla has picked ProtonVPN for its integrated VPN testing. And they seem competent and privacy-friendly.
Also, whatever they did with ProtonVPN, there's no reason to believe that there's anything wrong with ProtonMail. That's arguably their core competency. And they arguably brought in Tesonet because VPNs were not part of their core competency.