FWIW, substitutability is a hard problem with algorithmic pricing, even for relatively stable inventory categories like big box construction retail.

I'd guess (with some insider knowledge) that (1) there wasn't any prebuilt link to adjust substitute pricing & (2) Depot didn't care enough to adjust pricing.

Both Depot and Lowes are in it for volume and long-term, which is why they don't do things like spike generator prices after hurricanes.