Your first link refers to the version 1 manual - as denoted by the /v1/ in the URL. That manual hasn’t been updated for some time, and in the time since, the manuals team decided to include /Opt as well as the Alt key. That’s why you can see the Alt/Opt equivalent in the newer, 3.5 manual link (we do the same for Ctrl/Cmd).
It was an interesting point of discussion, as on my own macbook (now an older specimen, hailing from 2011) the key is labelled just “alt” - whereas now I believe they show “option” written out in full. As Macs now seem to be firmly in the “Option” camp, we decided it was necessary to clarify the key equivalence. For conciseness, the key commands we show in the manual use the abbreviated “Opt” but that is indeed the key that’s being referred to.