[DoctorOetker]

>It's awfully difficult to say that there is equivalent perceived SPL for different frequency domains, even as a trained listener.

The snippet you quote doesn't claim comparing intensities at different frequencies.

He is comparing only perceived 1kHz intensities, (in the presence or absence of maskers at other frequencies, whose intensity is not subjectively being scored)

[givinguflac]

Ah, thank you for clarifying, I misunderstood but still have the same curiosity about their methods .