I’ve been using my 13 inch iPP with MKB on my lap (can’t use in bed because my unruly dog would scratch that gorgeous screen in a heartbeat) and find it perfectly stable. But maybe that’s because my preferred viewing angle for the screen is more vertical than others reporting here—at least when I use it on my lap.
Once again, it looks like this is a case of YMMV (your mileage may vary). I value reports like the one from OP but suggest that others who are considering the MKB try it out for themselves to see how it works for them.
As for the suggestion of using an Apple Bluetooth Keyboard instead of the MKB, that isn’t really a comparable situation. For one thing, you’d need not only the keyboard but also a Magic Trackpad and, for another, that arrangement might work well in a static use case but is virtually worthless when portability is desired.
I really liked my original MKB when used with my 2018 iPP. This new MKB is even better IMHO—the keys are better and the trackpad feels more responsive. The latter is a function not only of the revised keyboard hardware but also the improved speed of the M4 iPP itself. Bottom line, coming from a 2018 iPP 12.9 inch with MKB, the new M4 iPP 13 inch with MKB feels like enough of an improvement that I’ve decided it’s a keeper.