Not a fix, in this case just a work-around for your issue…
If I make some the Industries with the overlaps, might be a good idea if I include a message to tell players to change that setting to 2 Tiles to help prevent the problem.
For Industries that I make with 2 roads or 2 tracks, I will pace these along the NE and NW edges so players can bulldoze the slope easy.
Honestly, that is a terrible solution. Save yourself and players pain: Don't tell players that your set requires a certain setting when you can use a proper location check to not require doing so. Players will not read that anyway
And you won't have to answer support requests you can easily avoid.
What you copied from FIRS does absolutely nothing to solve or tackle the problem you describe. FIRS doesn't try to draw over the tiles between and thus doesn't care about height level differences. You should really look at what it checks. And what you
want to check. Probably something like
nearby_tile_height(0,0) == nearby_tile_height(3,2)
with appropriate offsets would do the trick. Or
nearby_tile_slope(-1,0) == SLOPE_FLAT
for some tiles with appropriate offsets to check the tiles in between:
The land under the industry being flat is required (you check that) - but that is not sufficient.
Additionally you either need to require to be no slopes on certain tiles adjacent of your industry.
Or you need to require additionally identical height for at least two tiles in the disjunct parts of your industry.