Were you there? Holding a dog down when it's upside down is called "alpha rolling" and is very controversial. Cesar Milan dog whisperer likes it but other "experts" think it is a really bad thing, ie can make any fear aggression worse. I'd be looking for a different trainer.trainer flipper her over on to her back and held her down, and then put her away from the other dogs to cool off.
If the other puppies were squealing and yours was not backing off, then I'd agree with this trainer that it was an attempted "maul" although it's not clear if it was a malicious effort or just puppy that needs better soft mouth training with other puppies.
She definitely needs lots of time with lots of puppies or slightly older dogs to learn to play nice. It is good she's not interested in biting you guys. There are some types of dogs that are completely good with their family and will never bite them no matter what but are quite happy to attack other dogs. I grew up with one of these a labrador bull terrier cross. As children we could take bones out of his mouth and sit on him, but I was told stories about nasty things he did to other dogs. Blue Heelers and Staffies are often like this, your cattle dog cross may be as well. I guess it depends if you trust it absolutely with you or if you want a dog that is "dog aggressive".