Fair enough, but these were solutions that worked without js, and they weren't dynamically rendering maps on the front or back end. They were just showing squares of pre-rendered bitmap, and the square boundaries were fixed. If your point of interest was near an edge it could be quite annoying, like trying to navigate somewhere in the gutter of a paper atlas.

Even if they'd had an API that took a viewport, the result would have been stitched together from bitmap tiles because that's what they had.

It seems like the "invention" of tiles for maps must have happened as soon as anyone starting using a computer to render maps to bitmaps. The Ordnance Survey wouldn't at any point have rendered the entire UK to a single bitmap (at least not a map with any detail). It would have always been tiled.

Edited to add: Actually, the invention was much earlier than that. Paper maps were tiled before computers were a thing. And this would naturally have carried over to computer-rendered maps.