When choosing whether to use the new LFC paths, there are a number of reason that LFC could be disabled, one of which is that the FontCascade uses the complex text path. On GTK, there is an override that results in the complex text path always being used for all text, which means that LFC is never used in LFC integration mode.
This has been fixed by r288070 allowing LFC integration to work with the complex text path. *** This bug has been marked as a duplicate of bug 235266 ***