Opensource

https://www.gnu.org/philosophy/free-sw.html#four-freedoms

What is Free Software? - GNU project

The four essential freedoms

A program is free software if the program's users have the four essential freedoms: [1]

• The freedom to run the program as you wish, for any purpose (freedom 0).
• The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
• The freedom to redistribute copies so you can help others (freedom 2).
• The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.

I don't know however if it is illegal to use the source code without having bought the game first, so I don't know if toothbrush is correct with their point.

Something that I find could prevent it from being called free or open-source software is the fact that you are not allowed to make derivative works for comercial use.

You may not alter or redistribute this software in any manner that is primarily intended for or directed toward commercial advantage or private monetary compensation. This includes, but is not limited to, selling altered or unaltered versions of this software, or including advertisements of any kind in altered or unaltered versions of this software.

-- https://github.com/flibitijibibo/RogueLegacy1/blob/main/LICENSE.md

“Free software” does not mean “noncommercial.” On the contrary, a free program must be available for commercial use, commercial development, and commercial distribution. This policy is of fundamental importance—without this, free software could not achieve its aims.

-- https://www.gnu.org/philosophy/free-sw.html#selling