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...