Capsizing when left alone in the water is not the usual measure of ship (in)stability, rather we typically use the magnitude of the righting moment when the ship gets out of vertical. Shallow flat-bottomed ships are quite unstable from a ship stability perspective, because of their relatively high center of gravity and their resulting short KG distance (see also https://www.marineinsight.com/naval-architecture/ship-stabil...).
When a flat-bottomed ship gets out of balance, the righting moment is fairly small compared to either a deeper boat with ballast or a boat with a keel. Both of those ships are much more stable than négo chin boats or indeed standup paddle boards. Whether someone is standing on top of them to manually keep them upright is not relevant to whether the boat itself is stable or not.
I know about that, but for me, that inherently made it seem like they meant the term from physics (https://en.wikipedia.org/wiki/Instability), as with rowing singles used in racing, which don’t have a righting moment/have a negative righting moment.
When a flat-bottomed ship gets out of balance, the righting moment is fairly small compared to either a deeper boat with ballast or a boat with a keel. Both of those ships are much more stable than négo chin boats or indeed standup paddle boards. Whether someone is standing on top of them to manually keep them upright is not relevant to whether the boat itself is stable or not.