The fellow that runs the site claims that his company serviced and sold Naks
for years. His FAQ seems pretty thorough. I don't see anything there about an
EQ difference. It also includes this entry, which again notes a design
difference
between Nak heads and others':

Q: Why does Nakamichi use different calibration tapes and are they really
necessary?
A: As the head gap is different than other brands, the tape must be specially
cut
using equipment that lines up properly with the Nak head. Using other tapes
will not result in the accurate alignment and bias/level calibrations
necessary to make a Nakamichi sound as it should.

This is simply incorrect. It has nothing to do with Nakamichi heads, or the
heads for any other kind of analog tape recorder, regardless of format.

The Compact Cassette format had standards established by Philips. One of these
covered track width and location. No company could manufacture a deck with
non-standard track widths and/or locations and expect to get a Philips license.

Furthermore, head alignment has nothing to do, per se, with bias and level
calibration.