“Until this battlestation is fully operational, we are vulnerable. The Rebel Alliance is too well equipped; they’re more dangerous than you realize.”
“Dangerous to ‘your starfleet,’ commander, not to this battlestation.”
Tarkin also underestimates them because he overestimates the Death Star’s invulnerability. The station doesn’t have any Star Destroyers protecting it, something Palpatine apparently took into account setting up the trap in ROTJ. And Tarkin ordering the base on Scarif vaporized doesn’t technically count as a Rebel win.
With likely no surviving Imperials on the ground to contradict his view of the situation, Tarkin made himself look good and everyone down on the planet, (including Krennic) was collateral damage. Even though he couldn’t know the Rebels were unlikely to take the base, (which they weren’t actually trying to do) he saw the opportunity to put his new toy to use.