We have to remember that the Mercedes PU deal has been in the works for 2 years now. It fell through at the end of 2017 due to McLaren missing a Mercedes deadline, to which McLaren opted for a 3 year deal with Renault instead. It was always going to be a stop gap, and gave McLaren time to rebuild the team and get them back up the field. Trying to think back to the Renault announcement, I distinctly remember getting the feeling it was a temporary arrangement back then, and that something else was to follow (building their own engines was my first thought).
Seidl has obviously had an influence in pulling the trigger on it very early, as design on the 2021 car has almost certainly entered R&D with the big reg changes coming. Knowing what PU will be in the back of it is critical knowledge, even at this early stage. The announcement of it to the public may have been forced with all these rumours building up momentum.