I haven’t seen the clip, but with no other context there is no reason to think that Sanders is being positive about the exploitation that undocumented people suffer doing brutal work (in particular in food production). He is just stating something that’s actually true - the exploitation of undocumented people is the foundation of the American food system. And if they were expelled en masse, then this system will be in trouble.

I don’t see pointing this out as implicit support of that system. Unless he says that later, of course.

Now, of course, not providing an alternative or working towards changing that system is the problem.