Investigation Boards are independent. They report what they find...to change anything w/o evidence, based solely on politics or outside pressure would potentially be criminal and go against the reason we investigate anything.
To put it another way: If you didn't report accurately what you found, you're potentially allowing whatever happened to happen again...I would personally not be able to live with myself if I did that and there was potential for the same thing to happen again (and have more people die). NTSB investigators are not political. They're highly-experienced and exist to prevent future mishaps.
To defend his point: If there was a shortage of controllers, it absolutely is "common sense", since there is currently a class-action lawsuit from when qualified individuals were denied jobs in favor of those that fit within DEI principles. There is a ton of information about that.
I have no idea if that played a part and I do not like anybody (let alone POTUS) declaring what a cause is before an investigation completes. But, I understand his perspective since his own regulations were rolled back under Biden and "near misses" increased exponentially in the past 4 years.
But, as I said previously...there is rarely just "one factor" that holds the totality of blame.