His share of said businesses is an asset, and they can take that. This would therefore strip him of the properties for even less than they're worth.
Remember, banks aren't charities. They didn't let Trump personally guarantee a loan, if they knew Trump could not pay it back. There are all kinds of loopholes, true, but when you get a loan from a bank, you sign a contract, and they can/will enforce it. Deutsche Bank knows Trump has assets and partial ownership of assets. They also know he can't just give them to Donnie Dum-Dum Jr and shrug, saying "well I guess I'm broke...until you look the other way". Those people didn't get where they were playing fair, but they also didn't get where they were by giving billions to idiots and getting nothing in return.
If dodging loans with bankruptcy was that easy, everyone would do it (and therefore the rules would change). Imagine if I borrowed $1000 in cash secured with a $1000 computer. I give the cash to @cubby and also the computer, go back to the bank and say "Well I can't pay you". Not only would they take me to court to get the money by completely legal means, I suspect I'd be criminally liable for fraud for giving away the collateral. If @cubby knew what I was doing, he'd be an accomplice, so odds are he'd give the money and/or computer back or he's in trouble too.
Trump had six businesses that lost a ton of value and they could not pay their creditors. He was partial owner, but he didn't sign the contracts on his behalf, but his companies'. That's closer to the loophole you asked about. But as long as Trump has assets, especially the ones Deutsche Bank told him not to give away or sell, he can't just walk away from this one and claim poverty. Nor can he simply hide it under the rug and hope for the best.
If, for example, Deutsche Bank comes to reclaim the loan and Trump declares bankruptcy, the courts will decide who gets what share of Trump's owned businesses. If he tries to defraud a bank, Deutsche Bank will bring him up on charges, and still get their money from his family and businesses. People who give out loans aren't typically forgiving sorts that will say "Well, I guess we don't get that billion back, womp-womp" and shrug. They say "you owe us that billion, and if necessary, we'll get it from your grandchildren, now open wide we're going for those fillings".