I can't imagine Amazon actually buying PLUG but you are right they are watching to make sure that their relationship only grows with PLUG and they are probably dictating things like the United Hydrogen purchase for aircraft solutions. I am pretty sure they knew about Giner ELX if not helped choose them.
I know they bought some suppliers (like the pallet moving robot company) but they were a very sharp focus and to some extent it helped their competitive advantage
Even if they had bought PLUG when it was just a fork lift parts supplier since those parts were (still are) loss making it wouldn't have made sense.
Amazon wants PLUG To grow so that the price of the H2 they buy from PLUG drops along with the price for the various types of fuel cell they buy - GenDrive for forklifts ProGen for trucks and ProGen HP for back up.
Having Electrolyzers from PLUG helps them consider setting up as a supplier of H2 to the trucking industry as a whole as well.
I could be wrong and Jeff Bezos's replacement may want to make a dramatic move. Imagine Spain or Portugal saying 20% of our natural gas network is now hydrogen supplied by Amazon.
It would certainly shift them into new markets. Maybe I should by AMZN as well as my SHOP. PLUG will grow faster than both from a share price viewpoint.