I would think if he has $400M liquid, he should be able to get a bond for the balance (maybe $150M?).

That's a big "if" right there. He probably doesn't have $400M liquid. If nothing else, at least $100M of whatever liquid assets he does have are probably already encumbered to support the Carroll bond.

What's interesting is that while he's right that it's difficult to generate liquidity from real estate on very short notice, this isn't entirely a short notice situation. The trial started in October. The court granted summary judgment in favor of the state in a pre-trial ruling in September. Trump and his legal team have known for half a year that a likely outcome of this case was an adverse judgment for more than $100 million.

Given that, his legal and financial teams could have - and should have - started the process of obtaining mortgages or loans against his real estate holdings, so that he would have the liquidity to bond the very foreseeable adverse judgment. Now, Trump is notoriously a "don't nobody tell me no bad news" kind of person, so perhaps he was unwilling to prepare for that contingency. But it does raise the possibility that he was unable to do anything like that. That he's so leveraged that while he might have a billion or more in net worth in his real estate holdings, it's spread around so many highly mortgaged property that none of those assets have enough equity to support an additional loan.

IOW, he not only doesn't have $400M liquid - he doesn't have $400M in mortgageable assets.