It's true, but not how you said it.
Teams can only have 1 player on a designated max extension deal(that's probably a more appropriate name for it, not sure what it is). Basically guys coming off their rookie deal can qualify for this. Teams can't have more than 1 of those contracts.
Brooklyn has Ben Simmons on that type of contract. Bam is on that type of contract. Meaning, Miami couldn't add Bam to a deal for KD as Bam would be the 2nd player on the Nets with that type of contract.