I think Dowitchers are not completely impossible to ID at any time of year, although I am not capable of such. There are, however, several things that I think are helpful. Long-billed Dowitchers, if they are well fed, tend to have a bulky swallowed-a-grapefruit look. The front-heavy proportions contribute to a slightly more vertical resting posture.
The photos below are of a male Long-billed Dowitcher, taken in April ‘20. It is thinner at the moment from lack of nourishment. Later it will bulk up. The bill is more uniformly thick over most of its length, whereas the bill of a Short-billed tends to taper a little more over most of its length. The prominent white under-eye crescent is a good mark for Long-billed. On Short-billed, there is usually little if any white showing.
In addition to the eye placement geometry that @RCD posted, Short-billed tend to have a steeper forehead. Perhaps the article mentioned that.
On my bird, the white edged barring on breast and flanks is diagnostic for Long-billed. Other plumages can be more difficult, but I think some experts can still spot helpful clues. In the second photo, the rusty barring on the tail is diagnostic for Long-billed.
I hope this is helpful. I wish I knew these birds better. If there was a course on sorting out Dowitchers, I would take it. It should include time in the field and also in class. And there should always be a wall handy to bang your head against!