This isn't about Mastodon doing microblogging stuff. They can do what they want. But when it comes to interoperability with other software using an internet standard protocol, this is about Mastodon supporting the specifications of that internet standard protocol.

https://www.w3.org/TR/activitystreams-vocabulary/#dfn-content

By default, the value of content is HTML. The mediaType property can be used in the object to indicate a different content type.

Note it doesn't say the default value of content is "text/plain with 'a' tags". It says "HTML". From memory, HTML supports a few more tags besides 'a'. That's in a different set of specifications, if you want to verify. Specifications are important for standard protocols if you want different things to work together.