I don't believe that delaying announcing transactions they just constructed would provide any value to the network. It's better to have them out there and provide people with more complete information about them.
I would assume they're motivated to cut down on their own tx fees. I'm not going to expect any market actors to act for the network's benefit at the behest of their own.
Granted they could still bloat the mempool in a single batch broadcast with low fees and RBF later where necessary. Delays aren't the only way to cut down on fees.