I get the impression that the defense thinks this judge wouldn’t be a good bet to get a positive SYG ruling. The prosecution could also use it against him in a trial if the judge didn’t rule for SYG immunity. In this case it looks like the risk is higher than normal not to get a favorable result.
Assume that the judge rejects the motion for immunity. That fact is forbidden to the jury, as best the court can do so. It is expressly NOT a consideration for the jury, and they will at least be instructed thusly. Further, the standard of proof for the immunity motion is "more likely than not." But, for a jury, they have to find that the self defense scenario stated by the defendant is "impossible" in order to reject it - the reverse of beyond a reasonable doubt. Said another way, the actual legal standard, at trial the state has to negate or disprove self defense, beyond a reasonable doubt.
Pretrial moving for an immunity finding is very low risk to defendant. I think not moving for immunity is legal malpractice. Notice how O'Mara makes it clear this isn't his decision?