The system originally quoted in this thread is not meant as an IRS:
LCR-100 Gyrocompass AHRS:
https://northropgrumman.litef.com/fi...tt_LCR-100.pdf
So while it basically is a low accuracy IRS meant to be used with GPS.
Even the other unit they have is meant for planes that usually don't have an IRS because of cost and weight:
Its cost effective and low weight design makes the LCR-110 the ideal alternative for classical IRS/INS. This is a major step towards NextGen and SESAR operations since it allows the introduction of the benefits of inertial navigation into aircraft classes usually not equipped with those systems due to cost and weight.
And for planes that typically don't have IRS, 12nmi / hour drift rate doesn't seem that bad.
(As far as i understand these units are meant to support RNP procedures where a certain RNP is required in case of GPS failure, so long term drift isn't a priority)