Mastodon might not be legally exempt, but depending on how much effort the UK government puts into enforcing this, large swaths of it might be functionally so. Most instances presumably arent hosted in the UK, and while some of those outside that country might block traffic there or be big enough for the UK government to order ISPs there to block them for noncompliance, theres a decent chance that some smaller, foreign run instances might simply ignore whatever the UK is doing, and if a UK user signs up to one of these, or uses a VPN to use one that does block the UK, and can still get the content from the rest of the network due to federation anyway, then the platform as a whole could potentially get away with ignoring those rules in a way that a single large site couldnt.