It's triangulation.
The plans Mamdani campaigned on are going to cost billions, and will require state and federal support to have any chance of success. He said as much himself and I imagine that's why he agreed to the meeting.
Hochul supported him verbally and now Trump has too (words are free).
From Trump's perspective, if he had done a presser where he immediately shits on Mamdani to his face, and vows to oppose him with the entire Federal apparatus, then that adds fuel for more people to potentially support Mamdani-like figures if for no other reason than opposition to Trump. It also gives Mamdani an easy target to blame if his policies ultimately fail in NYC.
In contrast, Trump comes out and says "NYC is great, it's gonna be great under his leadership, he can do it" etc. then later when things don't go so well in New York (which Trump is betting on) then it's easier for Trump to say Mamdani owns the failures.
Its the same thing as building up a debate opponent prior to the event and then it becomes easier to suggest they underperformed expectations.
The AR-10 models from this time period do have hinge-open trigger guards. You drive out one pin and then the bottom part of the trigger guard hinges open to allow for cold-weather gloves to fit in and still be able to pull the trigger.