The aircraft is a reference, you missed that. They don't need to know numbers, just an estimate of how many, whatever is used for reference, distance. The actual numbers can be determined later. Granted, the higher, and further away the aircraft was, the less accurate the estimate, vantage point matters too, but we're talking about a supposedly low altitude pass at fairly close proximity. I'm pretty sure most people wouldn't make a 50/500 error when estimating how many aircraft heights it was off the ground, you could use wing span. I see a 747 on approach in the distance, I can quickly estimate how many wing spans it is off the ground. I don't have to know the actual wing span at the moment.