Bitwise didn't have many choices for porting.
Mozilla 52ESR, which would have been pretty well the end of the line for Mozilla based browsers though there are various forks that we could have followed, hard to say how hard that would have been. Dmitriy, like so many, seems to have burned out on developing Mozilla as well.
QTWebkit where they have experience and once done, any QT based browser shouldn't be too hard to port. As Falkon is the official KDE browser, it does make sense as the first to port. QTWebkit has the advantage that the QT people are currently it in sync with Blink, hopefully they keep doing it.
Blink, which is what Chrome, Edge etc are using. It uses the GTK toolkit (on Linux) which would have been a big job porting and would likely have looked ugly.
Apples Webkit, which no one besides Apple and those forced to use it uses.
At least with the QT toolkit, we get quite a few other programs and as long as upstream keeps it up to date, hopefully keeping our port up to date won't be too hard. The speed of changes in browsers makes it really hard for minor platforms.