It looks like you're using the Markua spec, not the Markua manual.

In the Markua 0.29 spec, alt-caption has been renamed to alt-title.

However, neither alt-caption nor alt-title have ever been supported in Leanpub. We will support alt-title in our Markua 0.29 beta when we ship it.

(We currently treat the stuff in the square brackets as the caption all the time, if present.)
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For the current version of Markua supported by Leanpub, please see:
​https://leanpub.com/markua/read
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