I followed this recipe and all went well until I was kneading my dough which was incredibly sticky even after a half hour of kneading and dusting...

It sounds like perhaps your potatoes had more moisture in them than the ones I use. I'd add more flour, but not too much, after kneading for a few minutes. Then I'd let it sit for a good 20 - 30 minutes to hydrate and add more, a bit at a time. The flax seeds also soak up moisture.
Lately, I've been using potato flour instead of mashed potatoes because it's easier to maintain consistency in the ratios needed to make the bread successful. I'm sorry the recipe proved difficult for you.
FWIW, when I saw the photo of the loaf that the Food52 editors made when this was selected as a wildcard winner, I expressed my concern that the recipe needed re-testing and correcting. (The pictured loaf just doesn't look right.) They told me no, that they would post as is and that the recipe could not be edited. I will probably post the corrected recipe as a new recipe and will link from the comments. I've done that before with other bread recipes that are in a similar locked stated here on the site. I greatly prefer mass measures, and have intended for some time to revise all my recipes to include them. I will put this one next in the queue. ;o)

The amount of flour necessary for a bread recipe is almost always adjustable. Many factors contribute to how much or how little you need. When the weather is humid, you often need more flour than a recipe calls for because there is more moisture in it.
If you are an experienced bread baker, you know that you want the kneaded bread dough to feel much like a baby's bottom. When the dough is stickier, more flour needs to be kneaded in.
Did you continue with the recipe and bake it anyway? and how did it turn out? Sometimes baking a sticky loaf yields a lovely crumb on the bread. But I'm guessing you need(ed) more flour.

Have you looked at the comments for the recipe? There's usually some problem-solving suggestions there. This recipe has been on the site since its earliest days, so I would expect some feedback if others have a similar experience. Also, you can go to Antonia James's page (one the recipe page, put the cursor over her name and click, look for a 'mail' icon) -- she's very experienced and a good communicator.