It makes a ton of sense if you look at it from the perspective of what leadership has been seeing: All this effort on maintaining their own browser engine, met with continued decline and further irrelevancy. Despite best efforts, a likely trend towards disaster.

In the face of that, it would be reasonable to think that the cost is nowhere near worth the effort, compared to just adopting Chromium.

I think it'd be very easy for them to think that their goals for the modern web are not intrinsically tied to having their own, completely separate software.