Unfortunately the argument is basically You're wrong. As someone who studied statistics,it's safe to say 1300 is a good sample,I or the other guy can provide resources for that. In between there's a division by √(1300-1) in the sample population which reduces the uncertainty a lot. Bringing in vague percentages doesn't help just because they are low. You'd have had a better argument if you were to tell how those 1300 were in fact,not randomly selected but there was a bias in selection,which you haven't done yet.

Telling "it's possible to have sampling errors" doesn't equate to saying the researchers in this particular study didn't account for it