These IBM folks need to have a chat with whatever department recently agreed to open-source MS-DOS 4.00 (IBM had joint control with Microsoft), because they know full-well that third-party copyright-free largely MS-DOS compatible products exist and have existed for quite some time now.

This is the same deal but with their bigger iron.

Now it's true that there were a few DOS clones that somehow fell afoul of copyright that were killed off pretty quick, but the only other way to get DOS-compatibility is by... reverse engineering.

And if they sued about that, they must have lost because alternative DOS clones continue to exist.

The only caveat I can see here is that the successful clones are open-source and free of cost.

If this company are charging anything at all, that could be the angle of attack. It might be the only angle of attack.

But I'm not a lawyer and have probably missed something blindingly obvious. Or devious.