Interesting...
I'm not too familiar in Bitcoin Core's fee estimation algorithm but my initial take is it's trying to trick the fee estimation by continuously broadcasting high fee replacement transactions.
Since it's a replacement, the each relayed replacement is cheap since it'll only need to add a few satoshi on top of the transaction that it replaced.

But I'm not sure if the fee of the replaced transactions that a node relayed wont be removed from its "bucket" and if those will still be counted to its fee estimation after the last replacement is confirmed though.
If it is, that could be the purpose of those rbf spam.