Welcome! The oil in coolant is a pretty common issue.
Unfortunately, some people just have bad luck. Read around here and be sure to ask any questions.
Welcome, no extended warranty for me but.. a few others here have had good luck with a few 3rd parties.. I will search around for you myself to see if I can find anything for you.
I would imagine the truck's ECM had an update available, and they went ahead and hoped that will fix your issue. I wouldn't bet on it personally..
But yes, if they continue to say ECM I would make them replace it..