Well you probably aren’t going to like this answer!
People lie. You should never take someone at their word for being clean of STDs. If you didn’t see RECENT results, I wouldn’t trust them. I work in the STD clinic for my local health department and deal with people being lied to by partners every day!
It only takes a single exposure to contract an STD. If you are worried, get tested! You will need to request a throat swab for chlamydia and gonorrhea, because a pharyngeal infection isn’t going to show up in a urine test. I recommend you just have a full STD panel completed that will include syphilis and HIV testing, too. Syphilis is on the rise and in some people can cause symptoms in the throat and mouth.
Although a bit riskier but if your oral health is fine and there is no deepthroat then the risk is very low to none.