It’s not Tesonet, Proton is wholly self-owned and managed. Proton VPN was briefly sharing employees with Tesonet during initial app bringup, and that partnership is long over.
Naturally due to competition and the huge importance of privacy in this space, people still bring this up, but Proton VPN does not and never will sell or share your data with anyone.
Then the question becomes why would they help a company that competes with their portfolio companies. And even stronger case that they got some shares in return.
One reason is that helping contributors that you know will survive (as proton was already well known) grows the market. Basically give the competitor of piece of the cake because they will enlarge the cake for everyone.
It's when your director goes and tells you not to tell anyone but to report to another building to another team for a month or three, and help them with whatever they ask, because you have a very specific set of skills. (In this case, setting up VPN backend infra.)
Source: I am a Proton VPN employee.