Semantics, really. The difference is in how it sounds - like the difference between "parenting time" and "visitation".
It sounds like the mediator could tell that BM was having a hard time with the idea, so he changed the language to make it sound a little better for her, but without actually changing anything.
There is a significant difference between sole custody
and primary residential parent. It gives the OP less control over the situation. It very likely means that mom will get more parenting time up-front, probably unsupervised, and the step-up plan means that there will be provisions to increase her parenting time as time goes on.
Sole custody is just that, one person has all the custody and perhaps there are provisions for very limited "visitation" to the other parent.
As an aside, it has been my experience that in probably more than 50% of the times, mediators, parenting coordinators, and custody evaluators will tell you one thing in sessions, and then blindside you with something different when the final report comes in. It's not as uncommon as one might think.