I'll read the link shortly but I thought they moved to signal because of leaks...from members of the group in questions. Not from someone hacking their handset or capturing and de-crypting messages in flight?
It will be hard to stop the leaks regardless of the platform, as screenshots are a common way to save and share conversations in this community. The reason provided by the Party, as stated in the article, suggests that it is due to group size limits. All other reasons for the switch are open to conjecture.
A Conservative spokesperson said the real justification for their MPs to use Signal was operational, rather than for security reasons. With so many Tory MPs elected at the last election, it had become impossible to fit them all in a single WhatsApp group, because they are currently capped at 256 members.
I think even that reason is BS to be honest: The tories won 317 seats in 2017. They "lost" about 10 people who were temporarily suspended from the party by Bojo. But I don't think they ever dropped below 256 MPs.
I wondered if telegram has some feature where you get an alert if someone screenshots the window (I seem to remember snap chat had this?). Do you know?