I work at Google (not in Calendar, sorry), and that doesn't sound likely. Google is perfectly capable of making mistakes, but "not bothering to re-code" would be out of character. A bunch of missing U.S. holidays would be a pretty glaring error, and bad PR, which normally elevates things to top-priority-for-fixing.

U.S. Thanksgiving shows up on my Android phone, with the color-code for the "Holidays" calendar (as opposed to my work calendar or anything like that).