For sure! I'm also skeptical that N or P are the limiting nutrients. A bit of conjecture, but my money is on iron! There have been a number of controlled iron enrichment experiments in oceans that lead to large algae blooms, suggesting most natural reefs are limited by iron. This doesn't necessarily have to translate to our home reefs but it feels intuitive.
As for corals, their growth may not be as hindered by iron (or maybe a different nutrient) as common algaes. This is a matter of necessary vs sufficient, i.e. it is necessary that both algae and coral need iron, but X amount of iron per day may be sufficient for corals and not algae. (My cracker example above was an attempt to illustrate that an organisms requirements for survival may be different haha).
Some more anecdote: I dose N and P in my tank to keep them elevated but don't often see excessive algae in my display. When I overfeed my tank with food, there will be a noticeable increase in algae for the next few days, despite N and P being reasonably stable from testing. I take this to be consistent that there is some other nutrient that is rate limiting and fueling the growth.
I'm tempted to want to look for ICP tests of people with scrubbers but these are just snapshots. It's possible that everyone has 0 iron (or other rate limiting nutrient) in their tank at time of testing, but that the transient spikes from feeding/dosing are sufficient for differential outcomes in people's tanks.