Your secret chats are already being read. Hell, secret chats are probably the first to be read.
A secret chat is only for local privacy, meaning if someone got access to your device they can't find them, nor will you get notifications (in case your partner is watching your screen).
Back when we just had SMS and MMS, they had secret chats, too. They were blatantly marketed as tools to cheat on your partner. GO SMS was one of the first to use them. Its not so blatant anymore, but its still the primary use for "secret" chats.
I remember reading an article that someone figured out everyone's GO SMS images (even secret ones) were uploaded online without any security whatsoever. You could put in the url and some random numbers and get any pictures and voice clips ever sent. And they never fixed it even after it was leaked.
So yeah, not so secret. You want secure.
Edit: Context: Sorry, my ADHD gets the better of me. I mean that there's more likely than not a government backdoor. Other users are unlikely reading your messages, but your messages are more than likely being scanned. Just be careful if you use TG.