I had to do some shenanigans to IKK a rouge flight a year ago (it was booked 150/150, but I wanted to cancel my existing ticket and rebook it), and I know for a fact I got the last seat.
I'd call in and ask them about it. Lower Flex fares being available does not mean they're not oversold, and an empty seat map on a route that sells lots of Tango also doesn't mean much.