ERC-20 51% Attack

I'm looking through threads to see if anyone has explicitely answered this question and it doesn't seem to be the case so hoping to get some clarification here.

If an ERC-20 token is built on the ethereum network and secured by it, does that mean its not susceptible to a 51% attack without attacking the whole ethereum network?

That's what it seems like to me, but wondering if there's any protocol dependencies that can be exploited that might make some ERC-20s susceptible.

Note: to simplify the discussion I'm mainly referring to the current PoW protocol, I know Vitalik has said that PoS makes a 51% attack unfeasible anyway