First, at the risk of stating the obvious, let's be clear about the question.  The question is not whether the business itself can be distinguished from its environment.  Obviously, a business has independent, legal existence within its environment and can be distinguished therefrom.  Rather, the question is whether such a distinction is possible for a business model and, if so, whether it is useful to make one.

Instead of addressing the question abstractly, let's take an example.  Suppose we want to create a business model for a supermarket.  Following Zachman, we will factor the business requirements according to the six interrogatives and examine the results.

What:  A semantic model ... concepts, terms, facts, etc.  We obviously have knowledge of customers.  Customers are in the environment, but our knowledge of them is not.  Inside or outside?

How:  Business processes ...  These are obviously our business processes.  However, they do accept inputs (cash, delivery orders, etc.) coming in from the environment (and paid-for packages of groceries go out the door).  Inside or outside?

Where:  A location model ...  The supermarket is definitely our location, but our drivers pick up from supplier locations, and we deliver to customer locations -- all out there in the environment.  Inside or outside?

Who:  An organizational chart with roles and work product connections ...  This is definitely our organizational scheme, but some of the work products (e.g., bad check returns) come in from the outside (out there in the environment).  Inside or outside?

When:  A business schedule, with events and cycles ...  The IRS requires quarterly reporting on wages and earnings.  OSHA makes unannounced inspections.  Events like that happen in the environment.  Inside or outside?

Why:  A business plan ...  This is definitely our business plan, yet it addresses risks (supply shortfalls, new competitors in the neighborhood, etc.) that are out there in the environment.  Inside or outside?

To return to the question, if you were to try to draw a boundary line between the inside (i.e., the business) and the outside (i.e., the environment), where would it be?  Even if drawing such a line were possible, it would clearly need to be drawn differently for each of the six primitives.  Would such a six-part boundary really help us much?  I can't see that it would.

Brief digression:  I read the other day that NASA is (still) trying to come up with a definition of life, so they'll know if they find it somewhere out there in space.  The question turns out to be a lot harder than you might think.  (One scientist commented that the exercise is like trying to generalize the class (or type) 'life' from a sample of just one instance ("life on earth").)  One of the proposed criteria is that any form of life we can imagine must have some kind of something (e.g., a membrane) to separate the inside from the outside.

That observation would seem to support the separation of business entity vs. environment position.  However, a model (e.g., a business model) is not the business entity itself.  A model is a multi-faceted representation of the business where interconnections across the 'cell' (legal?) membrane are often more interesting than the membrane itself.

A model does not need to be a mirror image of the real thing.  It is a view that as Zachman says, ignores (holds constant) all variables except the one(s) under scrutiny -- which may have nothing to do with where the cell boundary lies.  Any other approach is almost certain to over-simplify, a significant mistake for things as complex as businesses and all other living things!

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