Well, he’s wrong (or lying) about the computer not being covered because that has an 8 year/80k miles warranty on it.
Actually, there is a bit of murkiness behind what is and what is not a warranty repair when it comes to Subarus. (I’ve worked for 3 Subaru dealers for what it’s worth.)
Warranty will pay for a defect in materials or workmanship during the specified time and mileage framework but will not pay for what is considered normal adjustment procedures.
In other words, if a drivers door glass is allowing rainwater into the cabin because of defective window glass track then warranty would cover it.
If rain is getting in because the glass is out of adjustment then that would be (or should be) a customer pay procedure and warranty, extended or otherwise, should not pay for it.
(There’s a very volatile situation behind things like this that the customer never sees.)
I would also be curious about the story behind the 3900 miles on that Subaru. I’m not saying it’s a problem at all; just curious more than anything else.
Also keep in mind that the word “Certified” doesn’t mean very much in most cases. Like the use of Carfax it’s more of a feel-good sales tool.