https://www.law.cornell.edu/uscode/text/10/12406
Orders for these purposes shall be issued through the governors of the States or, in the case of the District of Columbia, through the commanding general of the National Guard of the District of Columbia.
As Newsom stated in his letter, the order was not issued through him, therefor not pursuant to the law, or in other words, illegal.
If the Feds are willing to illegally federalize National Guard troops, and the National Guard in any given state is willing to obey those orders, it tips the scales significantly. States need to start taking measures to ensure their National Guard is truly under their control. These are the "well regulated militias" referenced in the second amendment, and if they are subject to the whims of a federal tyrant, then the second amendment is moot.
In fact, they're already planned for this weekend. See the map below for the one closest to you:
https://www.nokings.org/#map