These blocks tend to be reactive, so if a blocked app starts using Tor, Iran will block Tor fairly quickly. So in return for a short-lived regain of the use of Signal, all of Tor gets blocked.

This proxy arrangement is better because folks who start them tell their friends in Iran, who tell their friends, but it isn't listed publicly, like most Tor entry nodes are. When the authorities find a proxy and block it, they only disconnect a subset of Signal users, who hopefully have other proxies they've learned from other friends or friends-of-friends. So now the blocking is trying to put out a thousand small fires that they have to find one-by-one.

<I have consulted for the Signal Foundation in the past, but not recently and haven't talked with anyone there about this>

Tor bridges work in the same way as this proxy; except Tor operates a centralized way of getting access to them.

Blocking tor exit nodes is considerably easier than an arbitrary proxy server. Tor provides a list, in fact.

No, it's the opposite—if Signal wants exit nodes, they obviously won't block them. It's the entry nodes that need to be blocked. Some are easy to find, but others require you to send an email from a unique email address from a trusted provider to get lists of IPs.

Using onion services doesn't use exit nodes, that's only for exiting to the public internet.