I heard on the radio at some point that it’s not great to eat after brushing, but it’s more important to brush your teeth at all. Unfortunately, they didn’t explain why, but from what I can tell, brushing your teeth with (normal) toothpaste does two things:

• Removes the plaque from your teeth, which is basically a layer of bacteria, which can damage the enamel, since they can produce acids. Or if you don’t brush for a longer time, they can form into hard layers which cannot be brushed away (so-called calculus/tartar).

• Repairs the enamel by providing fluoride molecules, which stick into places where the enamel is missing molecules. This is important in particular, since the places with missing molecules are more likely to get further damaged by the acids.

So, if you eat after brushing, you’re providing new food to the bacteria, which allows them to form into a new layer of plaque.
But it’s gonna take a few hours to re-form the new plaque, during which your teeth won’t get damaged as much anyways. And the new plaque sits on top of freshly repaired enamel, too.

This is definitely simplifying it, too, like apparently plaque can also affect the gums and obviously different people have more or less problems with teeth in general, but it still seems plausible to me…