If the bug I reported a couple of years ago hadn't been fixed, then even running a full node wouldn't have been safe. Basically, some older versions of Bitcoin Core didn't validate signatures in blocks older than 24 hours in order to speed up the initial blockchain sync, so they could be tricked into accepting an invalid chain where someone spent bitcoins that weren't theirs if it was far enough back. This wasn't considered a major issue at the time because you'd have to do a 51% attack for 24 hours to exploit it. If a large proportion of mining nodes are no longer validating blocks, that assumption is no longer true - you'd just need to mine one block, plus enough to push the non-validating nodes over the 51% line if they weren't already, and wait. (Oh, and I think Bitcoin Core may still skip full validation of older blocks during initial sync even with the fix.)
The idea that all these miners aren't validating blocks before building on top of them now is scary. Everything's designed around the assumption that they will unless they're trying to actively attack Bitcoin.
That's probably because no-one's produced a fully invalid block with sufficient proof of work, but supposedly the pools involved would quite happily have built on top of one because they're skipping block validation almost entirely.
The idea that all these miners aren't validating blocks before building on top of them now is scary. Everything's designed around the assumption that they will unless they're trying to actively attack Bitcoin.