Link to relevant announcement email: https://lists.zx2c4.com/pipermail/wireguard/2021-March/00649... There's also Jason's reply to apparently not-nice feedback of someone from NetGate: https://lists.zx2c4.com/pipermail/wireguard/2021-March/00649...
Netgate is weirdly hostile to a lot of opensource stuff, which should be strange given what all their tech is built on top of. This has been going on for years. (see opnsense etc)
Basically, all the opensource claims don't amount to a hill of beans, because you cannot compile pfsense yourself, even for their hardware.
(I'm sure someone could come up with the link)
The firewall should be the ONE place where this would be critical. You have to run their binary.
I also think it phones home.