Well that still remains the Caldari point of view of the effect of slavery on any economy, whatever is written in PF.
Not an universal truth on the matter.
There are a few different discussions going on in this thread and you've raised a few different questions so let me summarize the answers.
1) Why do players (not characters) use as a fact the argument that slavery is economically inefficient?
Answer: Because it is the consensus of economists, for a range of reasons discussed in this thread including lower economic growth due to lower consumption, lower labor quality, and lack of labor flexibility, that with certain very limited exceptions, slavery is economically inefficient.
2) Why do Caldari think slavery is inefficient?
Answer: Because PF tells us they do. Characters may use arguments developed by real world economists to support and develop that argument (just as characters use arguments developed by real world political scientists in debates over democracy, and real world religious scholars over the importance of faith) but ultimately, the authority for the Caldari in general thinking slavery is economically inefficient is the PF.
3) Do the Caldari think slavery is economically inefficient in the Amarr Empire?
Answer: PF tells us that yes, they do.
4) Do the Caldari think the Amarr should stop using slavery?
Answer: The Caldari don't think it's their business. They do think the Amarr would be better trading partners without slavery, so they may think in a general way that it would be good if slavery in the Empire ended, but they're not about to collectively tell the Amarr to cut it out. Individual Caldari characters, however, may be more pushy about it, as individuals.
5) Is slavery inefficient in the Amarr Empire?
Answer: Unclear. PF itself is contradictory, as comments about educated and compliant slaves (untrained and non-compliant labor being one of the great inefficiencies in slavery) are balanced by other references to this being an exception. We do not know enough about the Amarr Empire to know if they have solved all the various issues that make slavery economically inefficient. The Caldari, their allies, don't think they have, according to PF.
6) Does that mean the Amarr Empire are idiots for sticking with slavery/ do I have to find a way for slavery to be efficient for my character to not be an idiot for supporting it?
Answer: No. Slavery is a complex social (and in the Empire, religious) institution that, in human history, has generally not been used or supported for purely economic grounds, but for a range of complex motivations. Slavery obviously works
well enough for the Empire not to be a banana republic, economically.