Keep in mind a lot of these "leaks" are sensationalized, I think one of the past ones was legitimately classified but the rest were not. Not sure what is the case for this one in particular, but Gaijin doesn't allow posting any sources regardless. The game is full of inaccuracies to push people to buy their superior 60$ vehicles, and they constantly remove any proof from the forums so it doesn't make them look bad.
Edit: I dug a little deeper, like the other "leaks", this is an absolute nothing-burger, and typical gaijoobles cope. Here is that same manual for sale:
https://www.flight-manuals-online.com/product/eurofighter-typhoon/
Here is a dude submitting a FOIA request (and the associated response) to know if they're classified in the UK:
https://www.whatdotheyknow.com/request/confirmation_of_the_classificati
The response is basically "we don't know, ask the italians lol"
Either way, it's for sale online. If you look a little further it might find a pdf for free.