This feature isn't about security. Email is already pretty secure with TLS and DKIM. This is basically the equivalent of the "DO NO FORWARD" header people use for internal-only information but with a little more UX polish.
Anyone can screenshot or take a photo of any decrypted message. The question is when the email leaves Google's servers and whether you can trust Google with that same document.
Personally, if I had a message where I would consider a tool like this, I would just encrypt it on the client with PGP or something.
I believe Gmail already has that feature for corporate gsuite- greying out the forward button or something like that. Could be wrong. I know for sure Outlook has it.