I expect the parent is referring to human-caused melting of glaciers as the root cause of that tectonic movement, rather than the expected natural motion of the tectonic plates.
> As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland, where the land is rising at a rate of as much as 8–9 mm per year, or 1 m in 100 years. This is important for archaeologists, since a site that was coastal in the Nordic Stone Age now is inland and can be dated by its relative distance from the present shore.
The absolute value of gravity is (to oversimplify) the sea-level datum for current generation datums, like the delayed NGS (nominal year) 2022 datums.
They've (NGS) been flying absolute gravity measurement equipment in planes with precision GPS and lidar since ~2005 as part of the GRAV-D program.
Additionally, slight 'variations' (ie deviations from the expected circular/elliptical) in the orbits of GPS satellites can be used to infer slight variations in the gravity below.
Separately, the military likely has had all of this measured and modeled for decades as part of ballistic missile targeting, but remains mostly classified at useful resolutions.
The transitions from NGVD27 to NAVD88 to the work-in-progress 22 datum is exactly because the idea of a simple "universal" sea-level doesn't exist in the manor you would expect it to.
But even with the GRAV-D based datums, the actual observed water levels will occur at slightly different datum heights in different locations, for various geophysical reasons.
https://en.wikipedia.org/wiki/Relative_sea_level