It should be more resistant simply because a site finish is basically a single covering, when the flooring is installed correctly(no gaps/etc).

Solid or engineered hardwood?

Solid: Interior relative humidity has to stay in a tight range, otherwise gaps will open up. You might not see them but they're there. More variables than space to mention. Conditions when it's installed, moisture levels in hardwood and sub floor. It has to be in harmony, before, during and after.

Engineered: Far more tolerant of humidity swings, but can still gap (minuscule) with prolonged very low humidity. 20% and under.

Ken Fisher

Uptown Floors

In theory the site-finished hardwood will have a slight edge on "water resistance" than factory finished. As Handy Mac points out, the finish will coat the entire floor in a single SHEET of finish.

Now...that isn't to say it will be SUPER water resistant. It won't be. Most wood will have issues with large amounts of water allowed to sit on the floor. But the worry you have - water getting between the planks - is greatly reduced with a site-finish...again it is a large SHEET of finish that spans the boards = a thin layer of plastic = a little bit more water resistant than factory finished.

Again...these numbers and 'time frames' are very similar. But there *should be a slight advantage for the site finished. That's why we require our cork floors in kitchens be site finished after installation - the polyurethane spans the joints making the floor resistant to surface spills (many hours of protection because cork itself doesn't care...but the fibre board in the middle does).