openHABian can be installed on other non RaspBerryPi Debian/Ubuntu setups.
Quote from doc
is restricted to Debian/Ubuntu based systems
Hence, it is not 100% clear that you are using an RaspBerryPI.
Lighttpd does not get mentioned once in the openHABian GitHub repository, and as @mstormi stated, openHAB uses jetty.
Wait! Could it be that grafana, node-red or frontail (or another extra) does require lighttpd?
Nah I just checked all 3 repos, Grafana only has an example config for lighttpd.
I just checked my openHABian setup and lighttpd is not installed.