Security

I have not said about metadata, but contested your claims of conflating security with phone number identifier causing lack of anonymity.

Huh? My first comment in this thread did not say anything about metadata or anonymity; it was (like the linked blog post) discussing the attack surface that comes with using phone numbers for authentication.

It was you that brought up both metadata and anonymity when you said this:

Wrong. Anonymity from your contacts or phone carrier or government is different from security of messages and metadata.

(emphasis added). Phone numbers are also terrible for those issues, of course.

do you think organisations like Riseup are also backdoored

I did not say signal is "backdoored". I think their client and server software is most likely doing what they say it is, and Signal employees can probably honestly say they don't retain any data that they could give to governments. The backdoors, if you want to call them such, are in the phone number based design and the choice of company (Amazon) that they rely on to keep the promises that Signal makes to their users.

My understanding of Riseup is that they own their own hardware, which puts them in a better category than Signal already. They also don't require phone numbers. They do however use an invite code system to prevent spam/abuse, which they say they don't retain a social graph from... but it isn't clear to me how that system is actually useful to them if they don't. Unlike Signal, Riseup is explicitly for activists, which makes me reluctant to recommend it. I don't think it is intentionally backdoored and I think the people behind it mean well, but I think having a system explicitly for activists seems wrong as (1) it is a very attractive target and (2) merely using it can make you seem suspicious. The use of riseup has actually been cited as evidence of wrongdoing in an arrest warrant in Spain.