Agree that priorities aren't exclusive and there may be teams/individuals that aren't able to contribute if they stay in their current teams/roles
Where it becomes questionable though is when enough progress isn't being made on the top priority (reliability). If Github is being true to their word, they need to be pulling people off of teams that are working on features to work on reliability so that top priority gets the resourcing it needs.
Given the pace of improvement, and the cited example of moving to Azure from months ago, it's not super clear they are doing that. Also not clear that they aren't, maybe the move to Azure is just a more than 6mo project no matter how many people are on it.