Going further back, post
#212 has a quote of an earlier post from Noctem, saying:
"As stated before, actions in 5e CANNOT interrupt each other outside of specific examples like the spell Shield. You cannot use a readied action to interrupt another action. You cannot interrupt the Attack Action of a level 5 fighter in between his first and second attack with a readied action unless that readied action specifically states (like shield) that you can."
Since the examples given of readied actions clearly make sense only if your action can take place during part of someone else's action or move, he's then said also that the examples given of typical triggers are not examples of triggers the designers clearly intend players to be able to specify and react to, but rather, are examples of things a DM can rule either way on, with no implied suggestion that they ought to work as intended.
The "but it's a DM ruling" thing is a way to dodge around the actual original claim, which is that it is absolutely impossible for a readied action to occur between, say, the multiple attacks granted by the extra attack action.
I asked if they were arguing. Whether it was dodged or not that seems particularly past tense after so many pages.
With that much written down I've got an opinion on what the correct reading is, but necromancing that argument at this time feels inappropriate.