Follow up: This is the comment my astrophysics friends deferred to.

So I guess it’s down to noise/error, a medium-size black hole merger event, or my very unlikely crazy ramblings. Still, I might search for literature that provides a prediction on the likelihood (/frequency) and required experimental resolution to see events like supermassive primordial black hole births. I suspect it’s the frequency of observation relative to our short human timescales that makes such a detection very (very) unlikely. Universal expansion is at least partially to blame. But yeah, I'm almost 100% Occam'd, because there are simply so many more "medium" size black holes floating around than supermassive ones. Maybe millions or more more mediumbois for every supermassiveboi. Yep, "more more".

Intermediate black hole merger events might be much more turbulent/non-linear than the neutron star and/or small black hole merger events LIGO has purportedly seen thus far, and so may create a different "burst" vs. the previous "chirps". The consequence of the larger spatial scales of intermediate-size black holes, combined with a finite propagation speed of light/gravity/information, may produce a much more randomized gravitational wave signature. Mathematically, there might be analogs with a sort of resistivity based on signal propagation delay, inducing a stochastic process that then produces a relatively broadband wave signal.

Edit: Please ask questions?