What keeps the front camera microphone grill waterproofed?

If you cleaned it with alcohol, you might actually have destroyed it.

The top speaker opening of the phone does not block water ingress at all. Ingress protection is achieved with a duct secured to the glass with adhesive, then the speaker plugs in to the duct, there is an o-ring preventing water leaking through the plug into the phone. The speaker chamber is sealed, water may get into it but that’s it, just let it drain back out. The microphone is adhered to the duct, but it occupies a separate hole. The adhesive prevents water leaking out the sides.

The adhesive is probably less than a millimeter wide in some areas and alcohol may dissolve or weaken it.

The actual way to confirm whether there are leaks is to do a vacuum test, which is more complex for iPhone XS and later because the barometer is not in the sealed case, rather it opens to outside pressure via one of the bottom grills, so it is not possible to use it to test for pressure changes inside the phone when you squeeze the case, a special fixture may be needed to test.