The capability to use ";" for multiple index entries was intentionally removed way back in Sigil 0.71 in Feb of 2013 (so before my time).
https://github.com/Sigil-Ebook/Sigil...bc8313df0ba6e2
The comment to that change say it was removed to prevent invalid code generation.
My guess is they just forgot to remove it from the Sigil User's Guide.
I will make that change to the users guide since it was deliberately removed.