It's contextual. If it's used in a phone number, it's a pound sign. If it's placed before a number, it's a number sign. If it's placed before a tag, it's a hash/hashmark/hashtag.

No one would pronounce "#foo" as "pound foo" any more than they'd call a #2 pencil a "pound two pencil". Because "pound" is clearly not the right name in either context.

Americans have been comfortable using different names for the symbol in different contexts since long before hashtags even existed. So when websites started using them and referred to them as "hashtags", that was fine. It was a new context so it could use whichever name it wanted. (Well, "octothorpe-tag" is probably far too unwieldy to catch on.)

Of course if we're talking about the symbol without a specific context, then we have to pick one of the names. For most Americans, that "default" name is probably still "pound". Twenty years ago I'd definitely say that, but even then it wasn't ubiquitous. It wasn't uncommon to hear it referred to as a hash. And it seems like the use of "pound" has declined and the use of hash has increased as people now spend more time online and less time dialing phone numbers. There's also a generational divide with older people more likely to say "pound" and younger people more likely to say "hash".