Thock is just the inverse of clacky. I would describe "thock" as the sound that has a heavily dampened case and well lubed and quality stabilizers. Keyboard sounds are all the sounds your keyboard makes at the same time combined. So when you reduce those higher pitched tones that are distracting (often from wobblily keys/stabilizers, echoes in the case, etc) you are left with thick plastic hitting solid surfaces.
Any amount of plastic squeaking as the case flexes will contribute to that higher sound. Higher pitches require less energy to produce the same level of sound pressure than lower sounds do, which is why subwoofer amplifiers are 5-20x the power rating as speaker amplifiers.
If you want that deeper (thock) sound you need a desk mat to absorb the force coming from the case into your desk as you type, a quality case that doesn't flex or creeeeeek as you type on it, and case/plate foams can very much deepen the sound.