As noted here, the Iwbacklinks module returns the pageid as a string instead of as an int, as most other modules do. I've poked through the code, and don't see any obvious source for the problem, so it's possible it's more than just Iwbacklinks that's affected. (Note: I haven't done much PHP work, so I may have missed something that'll jump out to others.) The links below demonstrate the difference.
Iwbacklinks: https://en.wikipedia.org/w/api.php?action=query&list=iwbacklinks
Backlinks property: https://en.wikipedia.org/w/api.php?action=query&list=backlinks&bltitle=Main%20page