But isn’t Reforged using the same engine as the original, with higher res assets (mostly textures)? At least, I thought so, as that was part of the criticism of Reforged.

Nothing wrong with that, necessarily. In fact, I’d argue that’s exactly what I would want out of an ideal War3 reforged. Exactly the same game modernised for modern systems, with the “same” graphics just at higher resolution and detail.

Besides, World of Warcraft, StarCraft 2, and Heroes of the Storm were all built using upgraded versions of the Warcraft 3 engine, so it’s proven to be quite capable. It’s logical to use an updated version of the engine for an updated version of the same game.

Trying who port the game to an entirely different engine would not only be much more effort, it would risk introducing subtle differences in gameplay and feel that could mean it doesn’t play quite the same any more.

I haven't played it for a while now, but one of the major complaints after release was that many of the old custom maps didn't work.