I'm sorry I still don't see it. Your linked article says at the very top "There is as yet no unambiguous detection of an IMBH". Concerning gravitational waves,

I'm sorry I still don't see it. Your linked article says at the very top "There is as yet no unambiguous detection of an IMBH". Concerning gravitational waves, reference [7] says "We show that space based detectors such as the Laser Interferometer Space Antenna are likely to detect several of these sources". Reference [8] says "gravitational-wave observations could be used to accurately measure masses of black holes in merging binaries and probe the existence of intermediate-mass black holes". So no discovery yet, just theoretical work on what the signal would look like.