I don’t think it is new thinking, they couldn’t go either way once down at the river path. They go under the bridge and come out, they would be stuck and spotted by CCTV in the harbour, workers, folk on boats, as it is 24/7 in use. CCTV on road would also spot them if they did go under and then appeared on the road via one of the bases.
If they went the other way once on the river path, ie past the Boat Houses they would have been spotted by the CCTV there.
The path and area under the bridge would in general been very treacherous to walk along, so I doubt the walked far before deciding to do what it seems they did.
Especially if it seems they may have been worried about someone spotting / stopping them. (Conjecture)