What a bizarre conclusion.
Casbolt - imprisoned, to be implicated as only someone such as Poirot might implicate an absent suspect - told Swerdlow where to find the candlestick and implanted our trio's plot.

Swerdlow gave the candlestick to Duval, or at least led Mr Spiers to the study.

Then it was Duval with the candlestick in the study.
Or the lead piping in the study, depending on the degree of prior collusion.

Does Mr Casbolt even know Mr Swerdlow (of recent times anyway)?
I'm not even sure the conclusion is meant to be fully serious.

Rest in Peace, Max.