To expand on what others are saying.

Hash is one of the many names for #. Twitter and other platforms allowed you to use hash to tag your posts with a searchable keyword. Hence “hash tag” which gets shortened to hashtag.

People on these platforms may have had cause to use and think about # on a much larger scale than would’ve been common at that time. Sure you may be asked to press pound on a phonecall once in a while, but that never happened often enough that I could fully keep it straight from star. It was usually just stored in my head as “the special phone key that isn’t star”