This is a little different than using VPS services because with the VPS services you're actually connecting to the customer you appear to be connecting to, it's just very unclear who that is, because it's just one random VPS customer among a huge number of servers hosted on that same service.
There could also be an element of luck or differences in how strongly particular governments are trying to block particular apps and circumvention methods at a given moment. I know that varies a lot from country to country.
Edit: However, Signal does have a "Censorship circumvention" feature under "Advanced" options. So there is some level of official work on that from Signal. Have your friends tried this feature?
There are a number of countries gearing up to actively regulate instant messaging, including with potential encryption restrictions and app-blocking remedies for noncompliance, so I imagine this functionality will become more and more relevant over time. :-(
https://community.signalusers.org/t/signal-needs-to-shift-to...