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It wouldn't be impossible. There are like so many different certificate issuers, any one of them collaborating with a government would allow them to create a certificate that would be accepted by your browser.
I work in cryptography, and I guarantee if that's true "some person you know who worked in government security" would not tell you if they did know, or they are pulling shit out of their ass. There have been so many people that have looked at or worked on SSL/TLS implementations (including some of my coworkers), any vulnerabilities would have to be pretty subtle or clever, and that would be kept highly classified. Quit making shit up or repeating bullshit you heard.
That just would allow a malicious attacker to fake being the server, it doesn't actually compromise the TLS session. So you are talking about a much more sophisticated multi stage attack that needs to be actively executed. This wouldn't at all allow them to record traffic and decrypt later.
The certs authenticate that you are talking to the real server, the symmetric session keys that are usually derived from a diffie helman key exchange have nothing to do with certs. That's two separate (but connected) parts of the transaction to build a TLS session.