I tried to find the problematic harfbuzz part in Lazarus, but when i did a grep for the string 'harfbuzz', only two files were found: one is lazharfbuzz0.pas which has the bindings themselves to the lib and the other is lazpango1.pas, which calls in LazHarfBuzz0 in the uses list of the unit's interface section, but after that, it is not even used (as far as i could tell).
Also, these were among the gtk3 bindings, which is really weird, as Lazarus uses gtk2, does not it?