I have added baptism entry (sorry its a bit hard to read), saying David born to William and Janet RObertson. Not necessarily proof that its the same David that married Janet RObertson (2). Also, the IGI list David and Robert as sons of William and Janet, although dont provide any further details on David.
I guess this doesnt rule out the possibility that the David that married Janet (2) was never from Ferry Port - maybe just another David born in Edinburgh (or elsewhere)…
i originally found the record for David on ancenstry.com which recorded both Robert and David as sons of William and Janet - it also listed wives for both (David was listed as marrying Janet Robertson), and children were listed for both Robert and David. What I found odd was that for David it only listed all the children after John Robertson Edney, which left me with question of whether JRE was son of David or Robert. I don't have access to that anymore (my trial period expired and its a bit pricey), so not sure how that info was added.
ScotlandPeople (and IGI) also lists a David Edny born 1778 in Fife (Carnbee) and a David Eadney born 1787 in Dundee. For both, not marriage details recorded, although there are records for a David Ednie marrying someone (not Janet Robertson) in Dundee and another David Ednie marrying someone (not Janet Robertson) in Carnbee around 1825ish, so this possibly rules them out (see Unallocated Edney records. There are no other David Edney (or similar) in any records around that time (which doesnt mean that others didnt exist).
So I guess the arguement is that there was an initial lead on ancenstry.com, and so far there are no better candidates.