When banks loan against property and the collateral is the property itself, there should be an independent revaluation of the property to ensure the bank is not running into excessive risk by lending say, $300, on a property that is worth $200. In case the borrower default, then the banks sells the property for $200, but losses $100 because the property was only worth $200.
Trump claiming that banks never saw losses indicate that bank relationship managers were complicit in using distorted valuations to lend, potentially violating internal credit rules established by their own employers.
Another possibility is that Trump actually defaulted multiple times but banks do not publicly admit it.