This fork could have still been prevented even if only UTXO were stored.
The problem behind the fork here is that 950 out of the last 1000 blocks had the block version set to 3, but didn't follow through with their promise. By setting the block version to 3, they were voting to transition to BIP66, meaning that once 950 of the last 1000 blocks vote for version 3, then no more nonstandard DER signatures will be allowed in blocks.
Even if you prune spent transactions (so only keep UTXO) you still keep the block version, so you can still measure if 950/1000 blocks vote for version 3. And you are still able to verify that new blocks don't have nonstandard DER signatures.
