Why combining different factor types — not just adding a second credential of the same type — is what actually makes multi-factor authentication stronger.
"Two-factor" gets used loosely to mean "two credentials," but that's not what makes multi-factor authentication genuinely stronger. There are three fundamentally different categories of authentication factor — something you have, something you know, and something you are — and each one fails in a completely different way. The security benefit of "multi-factor" comes specifically from forcing an attacker to defeat several of those different failure modes at once. Requiring two credentials from the samecategory doesn't do that, even when it's marketed as two-factor.