That would be poor design. Are you saying that the Signal team would not be capable of implementing a more effective path? Other applications have followed this approach before; it's not rocket science.
If that was their intent, then yes, that would be an issue. The key is good UX; don't send the user there without explanation. A well-designed "read this!" screen is key, and even then you will lose some users. It's a trade-off.
Also, I did acknowledge that this approach will turn away "mass market" users, but again, I don't think that those users will ever be Signal's primary user base. Most people are going to use stock apps or whatever is most heavily marketed (read: whoever spends the most dollars on acquiring users). Signal frankly can't afford to buy its way into the mass market. It's a niche app, and it should focus on catering to that niche.
1. User chooses "attach photo."
2. The user hasn't installed the plugin. Who does that? So signal doesn't work.
3. The user sends their contact a facebook message saying "signal isn't working" and attaches the photo.
4. Both users uninstall signal and tell everyone who mentions it that it can't even send picture messages.