There are people in the world who don't have a phone. amazing eh

And yet they have a computer?

Yes. In the USA, people use the free library computers. In foreign countries, people rent computers in cyber cafes.

kids at home could have tablet without SIM and use signal to communicate with parents away from home

It's pretty common to buy/beg/borrow an old cell phone and just use it over wifi. Especially kids or anyone who is tight on money. Food/water/shelter ranks much higher than cell service and monthly server + Data plan (required for signal) isn't cheap.

So Signal isn't for everyone, because it's not for people in China who disagree with the Chinese government.

Requiring a phone number means it's not for everyone, period.

The article you linked requires them to have access to the old phone.

There was a post a few weeks ago about how homeless people struggle to retain their phones, going through a new phone every week [0].

Personally, when I travel, I don’t always have my US phone number available. If my phone is stolen while I’m abroad, I can’t get a new phone or recover my phone number until I return, which could be months later.

Signal is not for everyone and them claiming it is makes me trust them less.

[0] - https://news.ycombinator.com/item?id=33120534