Are "Waterproofing Primer" and "Grout Sealer" the same?
I'd doubt that the primer could be used effectively as a grout sealer.  The primer is designed (in part) to mix in with another liquid to aid in waterproofing.  Grout sealer is designed to soak into dry grout to make aid in waterproofing.

Grout sealer is meant to be re-applied once a year (I don't know how many people actually do that).

If it works for you, then its fine.  but, if you get a leak into the grout and under the tile you risk water stains and mould.