From
this video comparing the PC original to the remaster, it does look like
some textures were upscaled, but very few. The ones that were certainly don't look like they were upscaled using something like gigapixel, but there are some slight improvements, honestly though if the textures were able to be modded any community AI upscaling pack would likely look 10x better.
The AA implementation is quite strong in the remaster as well, but it does cut down on the pixel shimmering which was quite bad in the original - a sharpening filter would help quite a lot I would think.