Slow speed skills absolutely cross over. I've spent nearly a decade riding a bicycle all year round in central canada where ice and snow happens in november and stays till march. Those winter riding skills crossed over to this year's experiment riding a KLR all year. I'm totally familiar with taming lateral acceleration and what the edge of traction feels like on the icey pond because of the mountain bike. Picking a line in the dirt is a good skill that crosses over too. And committing to a maneuver. Like on a mountain bike riding fairly technical offroad stuff, if you don't fully commit to that hill climb or log crossing, you probably won't make it. Same on the motorcycle. And things like looking far ahead make you more accurate on your line. My road reading skills also seem to carry over, knowing what kind of traction I can expect out of a surface by looking at it, especially a snowy or wet one or woody or rockey or gravelly one. Eg. you don't have to slow right down to do gravel, but you might need to if you want to turn. Hard packed gravel is fine, but loose will be squirely. Ice means your front brake doesn't exist, don't even bother. That kind of thing is almost identical except with what you can do with your rear wheel especially at those "most accidents happen under 30mph" speeds.
A bicycle is so much more agile it's super for demonstrating countersteering to the unbelievers and how countersteering initiates lean angle and turn.
The one negative I find is when I was learning a motorcycle, I used to stick my knees out and wave them around to try to affect my CG like I can on a bicycle. 1) the knees do nothing to affect the CG on a motorcycle, there's just too much weight to fight against that way and 2) you lose all that good integration. The instructor yelled at me a lot about that.
I would agree that it's a big jump on the learning curve but the skills diverge after that.