If you're going to cast aspersions about hidden motives, you might as well cast them at the right entity.
If that's the case, it doesn't matter which megacorporation sponsors the work; it benefits all of them.
I'm not saying this specific security initiative is or isn't worthwhile, that remains to be seen. But whether it was intended or not, it's a windfall to Microsoft for an advocacy website to demand PEP740 when? and the most practical way to apply it right now is Microsoft's proprietary offering.
Also, if I'm reading this correctly, the docs warn you off setting up your own Trusted Publisher and steer you towards using an existing one... of which there is currently only one choice, Microsoft. That's just an endorsement with extra steps.
As I've said about 50 times in the last day: the only reason GitHub is being used for examples and was the first one enabled is because that's where the overwhelming majority of PyPI upload traffic comes from. It's not a commitment or an endorsement; it's a strictly strategic move to help the largest uploading demographic first.