I'm still reading the article, so perhaps I am missing something. But it seems to me that Feynman was perfectly entitled to think in terms of point (really line) charges.
By Guass' law, an isolated cyclinder with constant voltage will look to the outside world exactly like a line of constant charge-density. One cylinder among many will be slighly different, because the corresponding line charge will have external voltages superposed. But as the radius of the cylinder approaches zero, those will vanish in proportion to the 1/r voltage from the central charge.
Now the author might have some other way of getting to the same result. But that doesn't mean Feynman's argument was wrong -- it was just different.
I think the most important part of the question is whether the field inside decreases exponentially or linearly with the distance between wires. To the extent that Feynman didn't incorrectly answer with "exponentially", he wasn't wrong.
However, whether the wires have constant charge or constant voltage (across the cross section) is not just a matter of the argument being "different". As the author explains, if you take the wires to have be point-like (in cross section) then Feynman is right in taking the charge to be constant. However, if you want to discuss the scenario where the wires are not point-like, then you have to pick: do you impose that your wires have constant charge or constant voltage? You take ideal conductors to have constant voltage across, and the charge distribution is whatever comes from solving the relevant equations.
But I'm open to be shown to be mistaken though :-)
It's absolutely true that fat wires will behave at least a little differently from the point-like wires.
But if you are looking for a physical intuition behind the general mathematical form, then the thin-wire limit where you start. Big-wire deviations are an advanced topic, fit for engineers.
N.B. there's a difference between "thin wire" and "point like". I am saying that real wires, with constant-voltage surfaces will _behave_ like point-like charges as they get smaller.
Perhaps I read this wrong, but isn't the opposite story the case? As wires become more point-like, the effective shielding drops to zero. However, when wires become precisely points, shielding becomes perfect?
It dosen't drop to zero. It is worse than for fat wires -- but the maths is easier.
It's intutively obvious that fat wires should shield better (there's just more shielding). But the original author is right that it the explanation of why this works is lacking from the Feynman point-like appraoch.
By Guass' law, an isolated cyclinder with constant voltage will look to the outside world exactly like a line of constant charge-density. One cylinder among many will be slighly different, because the corresponding line charge will have external voltages superposed. But as the radius of the cylinder approaches zero, those will vanish in proportion to the 1/r voltage from the central charge.
Now the author might have some other way of getting to the same result. But that doesn't mean Feynman's argument was wrong -- it was just different.