For those who believe in Milei, this makes a lot of sense. It's not more clear to me why he chose to put everything in one DNU. He is basically say, you elected me to turn things around, so let me do the job I'm elected to do. If the legislature rejects the DNU, then he is saying, let the people confirm whether they still want him to do the job he was elected to do. Even though the referendum called by Milei is not binding, the lawmakers would face the blame for further deterioration of the economy if they go against the will of the people. Their reelection would be in jeopardy because Milei and their future opponents will certainly use that against them.
The safer bet for the lawmakers would be to let the DNU take effect if the referendum is approved. Whether Milei succeeds or fail in turning things around, they can claim they followed the will of the people, so they either take partial credit of the success or lessen the responsibility of the failure. If the lawmakers go against the will of the people, they take most or at least equal blame if things gets worse. If things stabilize or get better, Milei gets all the credit.
If the referendum fails to pass, well, then the Argentinians have no one to blame but themselves for what's coming and if I were Milei, I would just resign. In fact, he should hint that's what he would do.