i found what it was
/etc/alternatives/x-www-browser
has a variable want_touch_pinch set to 1 by default. this was triggering the adding of the --enable-pinch
no matter what you set anywhere else, it was being added by this script and over-rides the disable flag
setting it to 0 solved the issue.
perhaps a good question now would be - why is this script even messing with --enable-pinch ?