It is absolutely barely used.

I say this as someone who used it exclusively for 5+ years before giving up and using WhatsApp.

There is hardly even a comparison between Signal and WhatsApp.

Downloads != users.

Compared to the other examples, it's rarely used. For instance, Messenger has 3B MAU whereas Signal has 40M, which is just a bit over 1% of Messenger's user base, yet Messenger wasn't mentioned in OP among others which is my point.

Seems like you're splitting hairs. His point was that some of these services popular with the HN crowd aren't as popular elsewhere. And Signal has historically had a reputation of being used primarily by techies, not the general masses (even if the current numbers indicate that's shifted).