Basically they’re arguing that the case against him was built partly on evidence from his time as president. I have no idea why the prosecutors have agreed to go along with that. Best guess is to keep things in the same case rather than have it go to a different trial/appeal?

Although the Manhattan case does not center on Mr. Trump’s presidency or official acts — but rather on his personal activity during the 2016 campaign — his lawyers argued on Monday that prosecutors had built their case partly on evidence from his time in the White House. And under the Supreme Court’s new ruling, prosecutors not only cannot charge a president for any official acts, but also cannot cite evidence involving official acts to bolster other accusations.

…

In response to the letter from Mr. Trump’s lawyers, the district attorney’s office wrote that prosecutors did not oppose Mr. Trump’s request to delay the sentencing. "Although we believe defendant’s arguments to be without merit, we do not oppose his request for leave to file and his putative request to adjourn sentencing pending determination of his motion,” wrote Joshua Steinglass, one of the assistant district attorneys who tried the case against the former president.

Source: NY Times (Gift Article Link)