Some live ISOs like TailsOS do allow persistent storage without installation, which was (probably?) what they were trying to ask.

It still does, but they did add a username system so you don’t need someone’s phone number to contact them.

The lack of PFS and 256bit encryption in Session is a little concerning, and one other advantage of SimpleX is that it has no unique identifiers at all while Session uses random IDs and Signal requires a phone number.

I would also use Signal instead, but Session’s situation is not that bad unless one needs to accommodate the CIA or Mossad as part of their threat model, in which case I think SimpleX would be a better option. This is also in the article you posted:

Edit: I didn’t notice that the original commenter had said they “switched from Signal” at first, which is definitely worse.