Is a strong block cipher usable as a strong sponge function?
I believe that, in this specific case, you are correct; it would appear to take $2^{60}$ effort to find a collision in the above function.

On the other hand, there is one nit with this approach: this makes stronger assumptions on the block cipher than is typically assumed.  A block cipher behaves as a random permutation if it is keyed by a random unknown key; there is no such requirement that holds if it is keyed by a publicly known key.  AES is believed to act like a random permutation even with a fixed key, however that might not hold in general.  You could come up with an artificial block cipher that meets the normal requirements, but doesn't work as a sponge function.