What is the IUPAC name of iso-octane?

It is used as reference for the octane number, so the reason is historical.
Strictly speaking, you are correct. However, from all the isomers of octane, 2,2,4-trimethyl-pentane is the most important because of it’s anti-knocking character in gasoline fuels. Specifically the octane number is defined:
A gasoline fuel with an octane number of 95 has the same antiknocking character as a mixture of 95% v/v of iso-octane (2,2,4-trimethyl-pentane) and 5% v/v n-heptane.