Agreed, if I had to be the one who dealt with the extra pizzas. But if there was someone standing outside in my yard in order to discard the 99 duplicate pizzas before they got to my door (a travesty in and of itself, but that's another discussion), I would think of the system as a whole as "exactly once delivery".
And, analogously, that does seem to be what Sequin's system provides.
If someone named Bob was standing outside your yard discarding the 99 pizzas, then Bob would not be able to guarantee that you actually get exactly one pizza. It's possible that the one pizza Bob tries to give you gets rained on before Bob can actually hand it to you.
Only you yourself can actually implement a means of discarding extraneous pizzas, any middlemen introduced into the chain just adds another layer of indirection that is susceptible to failure.
No, Bob standing outside of your yard is principally closer to you - as in, holding your hand - and can't fail to release pizza only after you got hold of it.
Of course the rain may flip bits in any system, but that would amount to destruction of you. You won't worry about pizza then, but your clone will spring up and get the pizza from Bob. And Bob himself is similarly protected from single-point-of-failure problems.
Of course the whole Bob subsystem may be destroyed, but that's already beyond the threshold of the problem we choose. Otherwise we'd have the external system to check if we'd actually got the pizza, and resend it again.
EDIT: that's probably the definition of "only you" - the Bob being that close. Agree then.
And, analogously, that does seem to be what Sequin's system provides.