Hi Taya, 
seagrass has been amended in the IHO INT-1 standard as parallel to seaweed but distinct feature. It has 2021/2022 been discussed with opeanseamap authors and was integrated in the amendments for the nature of the seabed according to INT-1 section J: https://wiki.openstreetmap.org/wiki/Seamarks/INT-1_Section_J

Landsat-8/Google do not play a direct role. Rather the commit is based on a comprehensive scientific work, which in itself has been using a combination of ground truthing and satellite imagary. Its product was published as open access work CC BY 4.0 on https://www.seanoe.org/data/00765/87740/. Attribution to the team of authors of the scientific work has been made appropriately both by inclusion of the tag attribution and in the tag source of the commit. The further work for OSM validation I commit as OSM mapper. 
If you see further problem here, could you specify this, so that I could approach the authors of HCMR with a detailed additional request? 
The puorpose is clearly the indication of protected vegetated areas of the seabed in coastal waters and this is in my view very much achived for skippers of leisure crafts and their anchoring strategies by visibility of seagrass in the OpenSeaMap, OpenNauticalChart, OSMAND and other OSM map derivatives. Such visibility is much reflecting the purpose of the original scientific work published as open access data by HCMR.