Perhaps the Mercedes works team and the RP management have seen the benefits of having/being a 'second team' in the manner of RB/Alpha whatever they're called and Ferrari/Haas. Not the stupidest thing in the world, and I always felt that despite their clear design and organisational brilliance, those 'second teams' were a clear advantage for RB and Ferrari over Merc, for exploring design options, testing alternative things without impacting the works team, young drivers etc.
Or MAYBE Stroll Sr. just sees it as a way he can one day get his son in Bottas' current seat