I had the same problem a few months ago, my mate was looking at some wiring in a food wagon and he rang me for advice on what to do next, he says there is a 32A RCD protecting the installation without any breakers and I said well if its a 32A RCD then it must be an RCBO so its covered but he was adamant it was just an RCD, when he sent me a picture and I looked a bit closer it dawned on me that it was indeed just an RCD.You make a good point, I wondered that after I wrote it.
If it said 63A or 80A etc the you would automatically assume its just an RCD without any other protection, its just the fact that it has a low value on it that I think loads of people will think its an MCB as well, same as the 20A and 25A ones that are also knocking about. Just doesn't make any sense to me to have all that different tooling to make different rated RCD's when a 100A would cover everything, the cost on saved metal must be minuscule.