It does. The point is that the system is set up to limit the bad consequences. You might (will) get wrong query answers, but the heap data won't get corrupted.
Again Tom, if there is an update based on index scan, then it takes the tupleid and updates the wrong heap data right? The only difference between normal index and thick index is to reach back to the same index tuple to update the snapshot. How will that corrupt the heap data? Did you intend to say that it corrupts the index data?
