I believe the parent comment is using founding engineer to mean early employee (not necessarily first, and differentiated from "founder"). 15% is certainly "founder" levels of equity, but it's all arbitrary.

Founding engineer usually implies that you’re getting paid because the company has raised funds or has significant income.

Getting 15% is only really possible as a cofounder or if you’re joining as the only engineer before they have money to pay anyone. I assume that’s why you said “sweat equity agreement”.

There can only be 100% total, so giving everyone from cofounders and the engineering team 15% would max out at 6 parties total, assuming no investors and the remainder goes to a pittance of an option pool for new hires. Unless you never plan to grow the company the math just doesn’t work.

If you joined pre-money and took pure equity as compensation while building the company from scratch then you’re more traditionally called a co-founder.