This info is from MARA:
"Is my transaction guaranteed to get mined by Marathon?
No. Once accepted by Marathon, your transaction will compete with other Bitcoin transactions for inclusion into a block. You will receive a personalized link to view your transaction's status."
What exactly don't you understand about mining and transaction priorization for a block?
Let's assume slipstream.mara.com works as intended. You submit your transaction to it and it will be held hidden and not visible in the public mempools. But of course your transaction fee competes with all public transactions available to mara.com and all hidden ones submitted to slipstream.mara.com. There's nothing fishy with that, totally basic facts.
That's all what you've cited in quotation marks says, nothing more, nothing less. It's just that YOU don't understand basic mining principles and prefer to chime in to all the stupid statements by zero-merits loudmouths here. Pathetic!
~~~
What a pile of LLM bullshit suggestions and absolutely none will help to secure a withdrawal of those low-entropy private key puzzles like #67. You would need to be very lucky that merely within very few seconds after you submit your puzzle withdrawal publicly a block is mined and better another one soon, too.
But because mining a block is a random process, you can't rely or hope that your transaction will only live for few seconds in public mempools of miners before a block confirms your transaction. It can take any time between a few seconds or maybe an hour until a confirming block is mined. The roughly ten minutes intervall for new blocks is an average over a difficulty period of 2016 blocks.