By default, Bitcoin nodes frequently exclude large and non-standard transactions from Bitcoin’s mempool, even if these transactions adhere to the Bitcoin network’s consensus rules. As a result, complex Bitcoin transactions are often delayed or unprocessed. To encourage experimentation and development on Bitcoin and to enable and expedite the processing of large or complex transactions that comply with Bitcoin’s protocol, Marathon has launched Slipstream.

Is this a reaction to Ocean filtering transactions?

Marathon is the first Bitcoin mining company to offer a direct transaction submission service.

Haven't there always been ways to get pools to accept transactions directly?

Maybe I am completely missing the point. I have no experience in this.