It's not quite as simple as that because the dynamics of black hole formation in the transitional mass range seem likely to be somewhat complex. When big stars age and cool they can undergo massive dynamic changes, such as the red giant phase and triggering a supernova. Current models for these changes indicate that stars in this transitional mass range are expected to 'blow themselves to bits' to put it bluntly and not leave a remnant big enough to collapse into a black hole.
This is where the mass gap comes from. Stars smaller than the mass gap are modelled to become neutron stars, stars in the mass gap are too unstable to end up forming black holes. Only stars bigger than the mass gap range are believed to have gravity fields strong enough to overcome these instabilities and undergo complete collapse.
That's the model anyway. The discovery of an object in this mass range calls those models into question and that's why it's such as useful and interesting find. That's the short version. In fact there are several possible models, and this will hopefully help us exclude some and refine others.
I agree the question of how it formed is a bit more complex, but what it is seems pretty straightforward.
Some plausible formation mechanisms include primordial black hole and neutron star merger. It could also be some other alignment of statistically improbable events. We've only found one so far, after all.
This is where the mass gap comes from. Stars smaller than the mass gap are modelled to become neutron stars, stars in the mass gap are too unstable to end up forming black holes. Only stars bigger than the mass gap range are believed to have gravity fields strong enough to overcome these instabilities and undergo complete collapse.
That's the model anyway. The discovery of an object in this mass range calls those models into question and that's why it's such as useful and interesting find. That's the short version. In fact there are several possible models, and this will hopefully help us exclude some and refine others.