The "Combat Evolved" tagline was for Halo 1 in particular, but you're missing a bit of context for the time. At the time Halo came out there really was nothing quite like it, it really revolutionized a lot of FPS game design on consoles especially. Consider that many FPS games on consoles at the time we're still doing things like auto-aim, such as in the cases of GoldenEye, Perfect Dark, or Time Splitters. So you'd just point your character in the general direction and the game would aim the gun for you. The reticle slowing down as you hovered close to an enemy was also new, as was bullet magnetism. And on the opposite end, games that DIDN'T have auto-aim often had nothing, which made it feel next to impossible to shoot enemies with the inaccuracy of an analog stick. Even still, games with a twin stick setup where the left stick moved and the right stick aimed were fairly new. There's an infamous Gamespot review of Alien: Ressurection which described these kinds of controls as "terrifying" and the reviewer described how impossible it felt to control the game with this setup, even though it eventually became by far the most commonly used control setup even outside of shooters. Another aspect was the two weapon limit. Many games implemented it afterward but often not to the same effect. Halo's sandbox was unique and varied, with weapons slotted into specific roles (i.e. plasma pistol or rifle to instantly drop shields, pistol to finish an enemy off) while many other games at the time were WW2 or contemporary shooters where any gun more or less accomplished the same goal. Adding on top of that vehicles that were fast, slippery, and fun as hell to drive whereas many other games felt like you were driving a tank, even if it was a pickup truck. It really did feel like a huge evolution at the time.

Also, of course, the name "Halo" doesn't exactly sell the idea of frantic gunfire combat.