Mexico is allowed some freedoms, but when the marching orders arrive, they will do their instructed job
Remember in the early 2010s when everyone thought that there are independent countries such as Japan, Australia, UK, S.Korea, EU countries? Well guess what, it was all a lie lol.
When the orders come, you should know how America's neighbours are going to act. If Mexico directly harms US national interests, you should know what's going to happen to it afterwards.
Now if their president snubbed one summit or another, who cares. Its just PR summits. Demented Biden shows up, says a couple of incomprehensible sentences, shakes some hands, and then at the end everyone makes a statement about strong neighborhood and friendship and other such bs. Then the western propaganda uses this statement for a full week to say "US is back! China better be careful" and other such drivel. Its all about PR, no strategic matters are at stake.
However for Marcos SCS claims, they directly conflict with China's national interests. There is a huge difference between bs PR summits and what Marcos is doing here