I checked the "hashtags" link and was met with the following: "Hashtags are a critique and a rallying cry to gather round. They are the protest banners of the openweb"
Nothing says protest banner of the "openweb"(?) like a convention invented by Twitter.
I didn't see any comments on this page, nor did the linked Mastodon accounts have much engagement, and that suggests the latter.
Also, both the number of and length of posts started increasing dramatically in 2024, which might just be a strange coincidence.
They're both a symbol-with-label text convention for users, and in each case it designates a topic and mechanism for users of the service to converge-on the topic.
#'s are for signaling root access, and bash comments.
edit: and also IRC channels.