Was the letter personal to you, or did it seem a standard one with your name and the exact amount of the increase added in?
I ask because if the vast majority of the tenants are on rolling contracts, and he has a lot of tenants, it could just be a mass mail and those with ASTs got caught up in it.
Schools do this with low attendance letters, famously. The worst case I heard of there involved a child who had low attendance due to their chemotherapy treatment for leukaemia. Sometimes, computer says yes when it shouldn't.
If you get on with him, and the letter was in no way reading as personal to you, then a text just to say, "Hi, just wanted to clarify if that letter was only for those on rolling contracts, or whether you're in a bit of a spot right now and want to amend the AST accordingly?" might be an idea.
If he responds to say yes, it's to you, it then opens the door to suggesting a new one year AST in the new rent, which is generous of you and makes his life easier when you aren't legally obliged to (as I'm sure he will realise!) and if it wasn't, you don't agree to an increase he'll almost certainly accept but that he never intended to institute to begin with.
A landlord with lots of properties knows a fixed term rental is not something he can adjust at will. Either he is pretty much back against the wall and badly needs to do this, or he never meant those with ASTs to get the letter, and it was for rolling contract tenants only.