I see that @Dr_Xorile has drawn the same conclusion,
it seemed to me that T and L were source of this confusion.
And yet while pondering about this antonym conundrum,
I find that 'cross the pond is used in DC and in London.
But if you read too rapidly across the word here:
You might happen to miss a stroke and instead, read the:
They represent a dearth and glut, the difference is clear,
Yet rhyme and look alike to the (too) hasty eye or ear.
Now, British or American, I'll yield I do not 'get';
But the answer to the last line seems to make it a sure bet.
If I would take the first of yours, replace it with the latter,
The least, that's absolutely none! and surely that would matter
To those who gathered to partake in this replete repast.
So that's my guess, no more, no less, and this line is my last.
As requested, an explanation in plaintext:
feast/least. "Two of a kind, remarkably similar": the words both look and sound alike. When you skim-read ("glazed over") the word 'feast', you might miss the cross-stroke ("one little straight line") on the f and instead read an l, resulting in least. And if you replace a feast with the least - the least being nothing at all - then "all would be gone, and many would be very unhappy".
Hint 1: the sound of the word should be more distinguishable in the UK/US than in the US/UK. I learnt British English and feast/least rhyme for me; I don't know how they're pronounced in the US, but have learned in the past that pronounciations can vary wildly. Hint 2: the little line that's missing isn't a letter of the alphabet on its own, but is strongly hinted to be part of the letter; in this case the cross-stroke on the f.