001 Game Creator was able to determine that it was an asset designed to be used with RPG Maker. I can only assume they simply in kind added the warning letting the game dev knows that many RPG maker assets are for RPG Maker game engine only.

A lot of game assets are sold or given away on different game engines for only use with that game engine. This is why you always need to check to make sure. Some companies out there do go after indie developers for using an asset in an engine they don’t have a license for. From my understanding, it was an issue quite a few years back on Humble Bundle bundles.

The Humble Game Creator Bundle from Jul 2020 is licensed for noncommercial and commercial projects without any limitation on what game engine they are used in. It would be quite odd if they were since they were bundled with 001 Game Creator.

Many of the assets in that bundle use to be exclusive to RPG Maker only. That is no longer the case as many asset creators are starting to loosen their license agreements and expand into use for other engines.