That 53 is the combination of your 18 and a potential employer match of $35k.
Not only the $18k pre-tax, plus employer-match, but the limit also includes post-tax contributions.
For example, I contribute the full $18k pre-tax, plus my employer's ~$6k match, plus I do another $20k of after-tax contributions that I subsequently roll over to a Roth 401k. The full set of contributions ($18k + $6k + $20k= $44k) must be below the $53k limit.
I would do even more than the $20k of post-tax-then-roll-over contributions, and thereby run right up to the $53k limit, but my employer's plan limits post-tax contributions to $20k/year. The $20k limitation is sort of a bummer, but I'm grateful that the plan allows it at all.