Yes. However, that 300 MB is reusable. Which means, that if there is 100 MB of real traffic, and 200 MB of free space, then you can first fill 200 MB with 0.1 sat/vB, and then you can bump it a little, by using full-RBF. Then, if you replace just a single transaction, and everything behind it will be invalidated, just because of changed "txid:vout", then guess what: you have yet another 200 MB to fill it again! Which means, that everything, which was sent before, was basically free, unless some miners will decide to confirm it anyway (which is not done by default).
Bumping the fee rate would cause the transaction to get mined, so it would no longer be free for the attacker.
Then, if you replace just a single transaction, and everything behind it will be invalidated, just because of changed "txid:vout", then guess what: you have yet another 200 MB to fill it again! Which means, that everything, which was sent before, was basically free, unless some miners will decide to confirm it anyway (which is not done by default).
This is the reason why BIP125 has multiple conditions for a replacement to be accepted:
3. The replacement transaction pays an absolute fee of at least the sum paid by the original transactions.