But googling screenshots of all of them, I can't find any use of this character in their rulers. It's all dots, numbers, and bracket symbols.

So I think the end of the article is right -- it's a delta triangle that, for various reasons, got corrupted into that shape:

> If even the actual Greek uppercase delta is, quite unmistakenly, rendered as a house, then the theory that DEL is just a badly formed uppercase Greek delta character with the bottom corners cut off (due to a lack of horizontal pixels) starts to seem more and more convincing.