Unless we know for sure bridge cameras were functional / properly aimed the evening of January 25, 2017 (and do not show DJ) then we should not rule out that he could have walked down a bridge stairway to the Three Rivers Heritage Trail or even crossed the bridge to the North Side.
I was not entirely convinced from the press conference that bridge cameras were functional and, to be complete, I did not hear anything that would lead me to believe they weren't functional / properly aimed. What makes me question the integrity of those cameras though is the recent history regarding Pittsburgh's surveillance cameras.
One thing to keep in mind is the Port Authority is touchy about the public knowing locations of their cameras. I suspect this is why such detail was not provided during the press conference. LE indicated they did receive video from the Port Authority, which could mean bridge-mounted and / or other surveillance cameras along the rivers, locks / dams and other river-related properties or, maybe not all.
I want to assume LE is honestly reporting DJ's trail ends at FDB because they did not see him on functional bridge camera recordings but something just isn't right about the lack of detail on that point.