Understanding support and contrast for a particular question

In this sentence:

“Sachie ______ the treatment plan of most doctors, arguing they will mix up half a dozen different substances in the vague hope that some of them will have some salubrious effect.”

I thought it’s a contrast because “arguing” shows Sachie is critical. But the explanation says it’s support because of the comma.

Isn’t “arguing” more important than the comma? Shouldn’t this be contrast? Can someone explain?

Arguing doesn’t bring contrast

What it tells us is that the person does not agree with the doctors treatment plans i.e the second idea is negative if you consider their disagreement with the plan the doctors have

The second half of the sentence supports the first half of the sentence (the comma), and so the first half will be negative as well

There is no contrast within that first idea, so the blank will be a negative word