I'm pretty sure this is not true. W3Reforged is using W3's engine at its core. I'm sure it had some tweaks to allow the changes they wanted, but it's still entirely based on the original W3 engine and the gameplay itself is mostly unchanged.
Source:
https://youtu.be/de-aCP1CXgc?t=310 (@5:10)
The main way that W3 differs from a "pure" remaster is that it doesn't just "upscale" all models and textures, in many ways it re-imagines a lot of the art and style.
When it comes to engine changes, D2 is quite similar - from what they've said in previews replies about a D2 remaster, a lot of stuff was coded around the maximum screen resolution (800x600?), so the engine itself needs to be tweaked quite heavily to actually allow modern resolutions. It won't be built from scratch, likely, but it will be a new version of the old engine for sure - quite possibly with more engine changes than W3R did.
With that said, I agree with the sentiment of not pre-judging D2R because of W3R. Just don't pre-order, wait and see. Worst case you'll still have your original D2 to play, and you didn't spend any additional money. Best case you are able to buy a very good remaster of a great game.