While technically true, the NEMA 5-15R receptacles are rated for use on 20A circuits, and circuits for receptacles are almost always 20A circuits, in modern construction at least. Older builds may not be, of course.

That said, if your load is going to be a continuous load drawing 80% of the rated amperage, it really should be a NEMA 5-20 plug and receptacle, the one where one of the prongs is horizontal instead of vertical. Swapping out the receptacle for one that accepts a NEMA 5-20P plug is like $5.

If you are going to actually run such a load on a 20A circuit with multiple receptacles, you will want to make sure you're not plugging anything substantial into any of the other receptacles on that circuit. A couple LED lights are fine. A microwave or kettle, not so much.