I wish I had an easy way to create and post a map, but I'll try to explain.
If you pull up a map of Dallas, the Trinity goes from NW (Lake Grapevine) and through Dallas to SE (E of the town of Ferris) and continues all the way to the Gulf. The water flows from the NW to the SE.
These remains - just like several others that have been found in the last year - were in the area in and near the Trinity river flood area that generally abuts downtown Dallas. Usually bodies are found just south of the I-35 bridge, but that is because the river bottoms are a secluded place to get rid of a body and almost never because someone floated down the river (I can't recall a single instance when that was the case, but in theory it's possible), and as there are high crime areas on both sides of the river south of downtown Dallas and bodies have been dumped there for years.
Although, this one was about a mile north of the I-35 bridge.
OTOH, for water to travel from Frisco Shops to the Trinity, it would go down a creekbed that is usually dry and that leads to the north end of White Rock lake, then down to the south end and over the spillway, down the creek and the river bottoms south of downtown Dallas, and will hit the Trinity on the south end of William Blair Park. That's almost at Loop 12 and I-45 on the south side of Dallas, and about 5-7 miles (my map-scale guess) south (and downstream) of downtown Dallas where these remains were discovered. That's an almost impossible trip for a body to take, but in the 1 in a million chance it did, it's still way south and downstream of where these (and most) remains are being found.
The flow from Lavon and Lake Ray Hubbard ultimately hits the Trinity too, but that point is SE of Ferris, another 20-30 miles farther downstream.
It's also relevant that these "creeks" that in theory are flowing to the Trinity are dry or little more than a trickle for most of the year, and are only full of water when it rains. It would be a real miracle for a dead body to actually travel very far, if at all, if hidden in one of them when they were dry and then wait months for a big rainstorm that's not likely to float the corpse very far.
Hope that helps.