Yes: “simple” can mean failing to recognize intervening variables

Exactly.

When I said your argument was not simple because you recognize unequal assets and access to capital, I meant that you recognize intervening conditions that affect what happens between the beginning and the outcome.

And yes, assets and access to capital can function as variables in your historical explanation.

For example:

Legal freedom → free labour → capitalist employment → economic advancement.

That is relatively simple.

But your fuller explanation is:

Legal freedom → free labour → capitalist employment
+ unequal possession of assets and unequal access to capital
→ different economic possibilities → different outcomes.

The bolded elements help explain why the same legal freedom did not produce the same economic result for everyone.

For example:

Former slave

Legal freedom:

Yes.

Land/collateral:

Little or none.

Access to bank credit:

Limited.

Former slaveholder

Legal freedom:

Yes.

Land/assets:

Often substantial.

Collateral:

Available.

Access to credit:

Greater.

Therefore, formal legal equality did not erase economic inequality.

At the same time, capitalism could provide opportunities for some formerly unfree people to work, earn, save, acquire skills, establish businesses and eventually improve their position.

That is precisely the duality you are trying to capture.