I think this comment is slightly disingenuous, I think it's more obvious that the aesthetics of any game (let alone engine) will primarily depend on the quality of the Assets. If you loaded up these quake 3-era meshes and textures into Unreal Engine 4 or something else you'd call 'modern', I guarantee you they'd look the same or similar.

The lighting isn't even that bad sans indirect illumination, which unreal only just got recently and Unity is losing due to Enlighten being shut down.

This reminds me of people comparing open source software with proprietary software on account of aesthetics - it's not the licensing model, or the game engine, it's the presence or absence of artists that makes the difference.