Yes, the sheriff is rather insistent on this point about Sierra having been the one to toss the bag. That suggests to me that they may be trying to explain the possible scenario where there are no fingerprints belonging to Antolin on the handles or the exterior of the purse, although DNA bearing a "strong association" to Antolin's was found in her purse.
That is the only logical explanation I can come up with, but certainly not the only logical explanation.
I don't believe Sheriff Smith meant that Sierra herself "tossed" the bag, only that items inside were not what she would discard (keys, money, etc.). Since AGT's DNA is connected to the clothes INSIDE the bag, it's difficult to understand why Sierra would "throw" her bag out the car while undressed (plus the temp in early am was high 40s). At that point, she may not have physically been able to do that.
JMO by listening carefully to the press conference and the reporters' questions.