I agree, their line width has been atrocious since the early days. It was as bad in 2005 as it is now. The font size on their primary topic page text on desktop is also too small.
Checking a well filled out topic page, something like George Washington, a typical line might be 160-190 or more characters across in the top section. In some places on the page it can reach to 210+ characters across. It isn't a very good reading experience at their present font size and at that width.
In the early days the lines would have wrapped earlier on a 4:3 monitor. Using a vertical tab bar fills in the extra space and improves the readability of most web pages on widescreen.
According to Dan Luu, none of the citations on that page actually support what it claims.
Amusingly, given the context, I haven't actually read any of those citations. However, he's pretty careful, so I take the claim seriously, though I'll ultimately withhold judgment. As far as I'm aware, no one has come back to point to a citation and show that it supports the result.
Check out the sources on this page for more details: https://en.m.wikipedia.org/wiki/Line_length