John Carmack has been fairly vocal about releasing the source code for his games, particularly the Quake/Doom series for as long as I can remember, going back to at least Quakecon 2004 during one of his keynotes there. I think at the keynote when they announced Doom 3, he even talked at length about eventually releasing that engine as open source, I think in 7 years (which eventually did happen). He is not the flag carrier for open source games, but he may be one of the most consistent at following through on open sourcing their base engine over the last 20 years. Before leaving Meta as CTO he was speaking about making sure the GO and Quest 1 were usable in offline mode with sideload capability when they become EOL'd. In my view this release of code is not at all surprising in the least.