From what I have read the Diem blockchain will be open source, designed so that anyone can build on it, and is able to scale so that billions of people can depend on it for their financial needs.
The Diem payment system is built on blockchain technology to enable the open, instant, and low-cost movement of money. People will be able to send, receive, and spend their money, enabling universal access to financial services.
So I get the they filed a trademark for "Diem" but that was the smart thing for any company to do. Diem is already a name being used in financial services/fintech so they can't put the term within financial services on full lockdown. They would have only applied for a trademark because this was what any big organisation should do.
But its quite clear they want this to be used and utilised in an open source way by the financial sector and therefore imo companies affiliated with their program will be able to use the name "Diem" so that users will know what service they are using.
So yes everything is a gamble don't get me wrong. But if Facebook wanted to put lockdown on a term they would not have chose a generic word that is already being used by a fintech company imo.