That does not mean it is actually installed. It just displays that this package contains this file.
Maybe you have installed libilbc-git ?
Btw i am also on the testing branch. 
Update
After an update i got this version also 
$ LANG=C pamac info libilbc
Name : libilbc
Version : 3.0.3-1
Description : A friendly copy of the iLBC codec from the WebRTC project
URL : https://github.com/TimothyGu/libilbc/
Licenses : BSD
Repository : community
Installed Size : 182.4 kB
Depends On : gcc-libs
Make Dependencies : git cmake
Packager : Daniel Bermond <dbermond@archlinux.org>
Build Date : 12/19/20
Signatures : Yes
Not really… better report it here: [Testing Update] 2020-12-22 - Kernels, Pamac 10.0.1, Firefox-Beta, Nvidia, KDE-git