The case for deterring authoritarianism is not G20 protests, it's
(a) groups like Deacons for Defence (https://en.wikipedia.org/wiki/Deacons_for_Defense_and_Justic...) where the presence of weapons serves as a deterrence, or if you want to turn to more grim times,
(b) events like Warsaw Ghetto Uprising (https://en.wikipedia.org/wiki/Warsaw_Ghetto_Uprising)
where the concentration of weapons counts for forcing an oppressor to suffer casualties instead of just rolling over the unarmed opposition. That is what the quote you are responding to alludes to.
Finally, weapons also serve as deterrent for the out-of-control forces, e.g.
(c) 1992 Los Angeles Riots (https://en.wikipedia.org/wiki/1992_Los_Angeles_riots) when the government is not acting directly or indirectly against a group but is simply not acting at all during critical time
All of these has happened before, and all of these will happen again, during our lifetime.
(a) groups like Deacons for Defence (https://en.wikipedia.org/wiki/Deacons_for_Defense_and_Justic...) where the presence of weapons serves as a deterrence, or if you want to turn to more grim times, (b) events like Warsaw Ghetto Uprising (https://en.wikipedia.org/wiki/Warsaw_Ghetto_Uprising) where the concentration of weapons counts for forcing an oppressor to suffer casualties instead of just rolling over the unarmed opposition. That is what the quote you are responding to alludes to.
Finally, weapons also serve as deterrent for the out-of-control forces, e.g. (c) 1992 Los Angeles Riots (https://en.wikipedia.org/wiki/1992_Los_Angeles_riots) when the government is not acting directly or indirectly against a group but is simply not acting at all during critical time
All of these has happened before, and all of these will happen again, during our lifetime.