It really depends on exactly what you're measuring and what your goals are (and thus, what the law does/should say). Is the goal to reduce the amount of fuel (petrol) used? Then yes, tax incentives should apply via whatever metric you're using. If the goal is to incentivize a switch to a different fuel source, regardless of efficiency, then no.
Many people with plugin hybrids only add gas a few times per year. The (small) battery is enough for them ~95% of the time. For most (but not all) purposes, that would count as an EV. I can see your point about them not being the same, and why it would need to be handled separately, but it feels too absolutist. There are certainly shades of gray in it.