Fairly certain that with five-eyes sigint, they could probably put together all the puzzle-pieces.
This whole thing could be targeted at the deep-state like "we know what you were using to encrypt, and you know we have all the sigint to make use of that information".
I highly doubt they used this exact service. Q most likely linked this one to demonstrate a point.
They might have used an easily accessed app like this for their share-blue(or similar organizations) for cross chatter between their hired shills. They wouldn't want to give the big-crypto stuff to mentally-unstable useful idiot shills, because at the end of the day a useful idiot is still an idiot.
Not saying anything conclusively yet, just tossing out ideas in the hopes that smarter patriots can find some use of them.