Athletes consider minor & independent leagues as "professional," and it is.
This is true for all college sports, not just baseball. When they sign LOI as HS senior they believe they have a shot.
rhobbs point (I think) is that HS seniors should play to their talents. If they are a high academic talent, they should not subordinate that talent to play DIII, NAIA, etc.
By the way, I watch alot of DIII.