Very true. In fact, you probably could have gone further and pointed out that, in many situations, it's the fourth level - since OPD-mediated ADS should (in TN systems) remove some risks of electric shock due to faults. However, ...I would point out that in most cases* the RCD is the already the third layer of protection against electric shocks.
True, but a substantial proportion of shocks probably result not from failure of those two levels of protection but because they have been circumnavigated by the victim (particularly if an electrical DIYer) who has exposed bare, uninsulated live conductors and then touched them. They have engineered a situation in which an RCD is the only level of (possible) 'protection'.The first layer of protection against electric shocks is the regular insulation ... The second level of protection against electric shocks is either the use of a second layer of insulation or a metal layer that is connected to earth.
In any event, RCDs provide (as well as other things) a conceptually totally different type of protection. All the other levels of protection you mention (plus OPD-mediated ADS) seek to prevent electric shocks occurring. RCDs may also do that but they also, uniquely, offer some potential 'protection' even if (despite all the other measures) a shock does arise. In that respect, they are the one and only level of (possible) 'protection', so if a single RCD fails to operate, one has no 'protection' of that type.
As I've said before, it's down to a personal view of the risk situation. If one is happy with the various levels of measures designed to prevent shocks, plus a likelihood (be it 93%, 97.2%, 99% or whatever) that a single RCD will operate if, despite the preventative measures, a shock does arise, then that's fine. However, if one is less than happy with the probability (whatever it is!) of having (any) 'protection' once a shock has arisen, then one should consider the possibility of a second RCD. It's a personal decision, which unfortunately has to be taken without much useful data on which to base it.
Kind Regards, John