The abstract doesn't mention that accuracy is better than GPS, but by INS (inertial).
It is not better than GPS. It is better than traditional inertial navigation systems (INS). But the accuracy is sub 500m for a good portion of operations vs multi-km resolution for traditional inertial systems.
The title should be changed.
Yep, the title is completely wrong. The actual article says ".. INS", not GPS. It can't compare with GPS to begin with. From the article: "the best final positioning accuracy we achieve is 22m". GPS can be accurate to a centimeter level, even inaccurate (no other reference) GPS is at least accurate to about ten meters.
For plenty of military applications, 90m accuracy is a valuable fallback in a GPS denied environment. It's probably not nice for targetting purposes. But for general orientiering and the question 'are friendlies in this area' it's a lot better than nothing.
MX with inertial guidance had a CEP of 90m.
This also omits how often the area needs to be resurveyed. Could be yearly, which isn't bad, but that could limit some applications.
Copy & Paste error, the "alternative" is missing from the title
This has been possible for some time, and there is an open implementation here: https://quantumvillage.org/.