The dealer has warned that the dipstick is frozen, and may break and cause internal damage if removed. The dealer is not required to cause this damage and obligate himself to repair it. The dealer has notified the owner of a problem that the owner will be responsible for, unless a warranty still applies.
What I can't understand is how the devil it got frozen in there in the first place. To my way of thinking its a manufacturing defect. I mean its plastic on the stick contacting the metal sleeve on the diff, just how bad can it be?
Unless someone inserted it with 200 ft lbs of pressure and maybe jammed the o-ring in a bad spot.
As regards the responsibility for repair, if the dealer can't service it, then its time to open a case with SOA. Most of us have pulled that stick a good number of times over the years and beyond the crud that can build up on the outside there's not much to worry about.
From my own experience on my 04XT I bought a PCV valve from the dealer and took one look at what it would take to install it and said "no way". So I brought it to the dealership and asked them to replace it because I couldn't see any way of doing it without breaking the attaching fittings.
So I advised them it looked fragile, and when they went to change it they broke it all to pieces. It wasn't a warranty item but they gave me a loaner car, and ended up replacing the entire assembly for no charge. So they do have leeway.
As to a dipstick that's not removable, Subaru needs to deal with this. Its an embarrassment to the mfg for that to happen.
They have a lot of wiggle room on what they can and can't include in the 5/60 powertrain warranty, as far as that goes so does the dealer.