hello there! I'm glad you are using the art :)
Here, it is probably due to your import setting. Maybe this will help:
https://youtu.be/WSjIBRMq46c
I don't see a flags panel ther
The flags panel has been moved in godot 4. On the tilemap node you need to change the texture type from linear to nearest
Thx found it
The video was in Godot 3, In Godot 4 you can just change the texture from Inherit to Nearest.