Yes, your mechanic was at fault. Transmissions are very delicate mechanisms and need the correct fluid. I’m also surprised it lasted 20k miles with the wrong fluid. Usually a CVT will fail much sooner with the wrong fluid.
You should go back and talke to him nicely, with receipt in hand, and also the dealer’s written diagnosis and estimate for installing a used or rebuilt CVT. Have the dealer do the work, but ask the mechanic to have his insurance co reimburse you for the work.
Your mechanic has no excuse for his mistake, so he should absolutely pay. First, he should know that ALL Honda automatic transmissions require special Honda fluid, not generic ATF, and second, he should have consulted his book and found that yours was a CVT that required CVT fluid. So he has no excuse and should absolutely pay for ruining your tranmission.
Again, don’t let him do the repair. Make sure it’s done by the dealer.