In one of the random contracts, named 'Repossession', the objective is to destroy vehicles, which are supported by a lance of mechs. Darius states that we may optionally destroy the mechs, but they're actually marked as a primary objective instead of secondary.

It's a bit of a running joke that Darius is the worst intel officer ever. Nothing he says is ever right and that in repossession contracts especially you are more likely to encounter Mechs than vehicles.

tbh i just ignore all that cause i find that in most contracts, its acutaly easyer to just bull though and kill everything then it is to hunt something then sneak out.
your right tho, on that job type mechs should be 2nd obj not 1st. were they with the vehicles in immidite area, like srounding\escorting them? or were they off a ways? in the former i could see why they would be listed as primary obj as well if they were tagged into the same squad structure.