It sounds like he’s moving on to a new project. This is speculation, but maybe he had been doing Standard Notes for so long that he wanted to find a way to get out of it. Now Proton’s management is good enough that he feels comfortable finally leaving to make a Slack competitor. Without more details I wouldn’t read more into it, but good catch for sure!
If anything it makes me think that it’s more likely that Standard Notes will integrate into the Proton ecosystem eventually.