The parameter moodlewssettingfileurl is used to indicate if you want the URLs in the content to be rewritten (to use pluginfile URLs) or not. But the WebService core_calendar_get_calendar_event_by_id always returns the URLs rewritten even if you set moodlewssettingfileurl to false.

I've seen that the calendar's event_exporter_base calls file_rewrite_pluginfile_urls directly, so it always rewrites the URLs without checking the parameter.