On the other hand, if you start classifying vehicles and carriers as "defining" Sentai mecha, I think it starts to lose its meaning. You'll be blending it with Metal Hero and Tomica Hero and any other show with teams and vehicles.
The formula for Sentai is color coded fighting team + giant robot. Not mecha. Goranger and JAKQ don't fit the bill there, but they don't have to. No genre springs into being fully formed. It refines itself and picks up traditions over time, and with time those traditions become an indispensable part of the genre's identity.
Whether or not they were there from day one (and you could even argue that they were, since Battle Fever J was the first series actually called by the title of Super Sentai), it's undeniable that the giant robot has become a longstanding Sentai tradition, and it would be unthinkable to do a modern Sentai without one.