That equivalence does not follow. The counterpoint is simple: can people figure out how to get burner numbers? Yes, they can: there are a myriad of services for doing so. And, importantly: journalists already know how.
Sure, you go ahead and buy an illegal burner number, then download Signal/Whatsapp from the Play Store and reverse engineer that binary to see if it "does what it says". Other people might find it useful to look at this comparison to discover alternatives that better fit the characteristics they find important.
Second, this is inconvenient: if you change your IP address or log out, the server will require you to provide a SMS code.