For a fan to make this a reality by smooshing the two together is wildly impressive. I imagine many of us have thought 'It would be so cool if this game had Feature X from Game Y'. Future plans include 'sieges, duels, new troops, inventory and equipment system, and much more'.
The creator explains on Reddit that battles and armies are generated by calculating the ratio of army sizes (given M&B2's 1000-unit cap), and trying to select M&B units similar to your CK3 army.
Well, now the first public version is out. You might remember seeing this earlier in the year.