VFIAX is a mutual fund. Vanguard holds a patent which allows a dual share class, where the ETFs and mutual funds share the same pool of securities. This allows share conversions from mutual fund->ETF, interestingly.
Not exactly a good argument for mutual funds being the same cost as ETFs though. It's only technically a mutual fund from what I can tell. I see your point though.
Vanguard's Admiral share mutual funds almost never differ in cost from their ETFs.
As for it only "technically" being a mutual fund: consider that VFIAX predates VOO by 10 years. How does that square with VFIAX only "technically" being a mutual fund, when presumably it was a real, honest mutual fund in the years 2000-2010 (before VOO was introduced)