Within a system, sure.. if you control force then slip angle is an output which will be contingent on the full set of tire states which include load.
However, the point still is that the definition "is what it is" and slip angle and load are two independent quantities used to define a state.
In any event, I thought we had pretty clearly answered the question some time ago. I'll be honest, I haven't followed what's really been said over the past several posts.
And yes, strad. Difference between orientation (how you point an object) vs the direction of travel (where it goes). However, sideslip doesn't have a preference for what medium you're working through.. be it driving across the ground or flying through the air.