I understand your question to be, "Does the Modigliani-Miller theorem have any relevance for forecasting the probability of default based upon debt to equity ratios?"
Not really.
The Modigliani-Miller thoerem is about the total value of the firm, not probability of default. As @n00b2 said in the comments, "Nobody, and certainly not M&M, ever said that the probability of bankruptcy has nothing to do with $\frac{D}{E}$."
The M&M theorem doesn't say that $\frac{D}{E}$ doesn't affect the probability of default. Rather, the M&M assumptions imply that bankruptcy doesn't matter in the sense that it wouldn't affect total enterprise value.
Quick review of Modigliani-Miller (M&M) theorem
The M&M theorem says that under the M&M assumptions, firm value is invariant to capital structure.
The basic idea is that if firm cashflows are taken as given and capital markets are rational, then the total value of a firm doesn't change based upon how those cash flows are split up between debt and equity (or in fact any type of security).
Under M&M assumptions, bankruptcy doesn't matter!
In an M&M world, there is no cost of bankruptcy, there are no lawyer fees, there's no loss of consumer confidence in the firm, etc....
In an M&M world, bankruptcy doesn't mean anything besides that cash flows are going to creditors instead of equity holders.
Example:
Let $D_f$ be the market value of the debt of Tesla if the debt has a face value of $f$. Let $E_f$ be the market value of the equity of Tesla if the debt has a face value of $f$.
Under the M&M assumptions $D_{\text{100 dollars}} + E_{\text{100 dollars}} = D_{\text{100 billion dollars}} + E_{\text{100 billion dollars}}$.
In the case of 100 of outstanding debt, virtually all the firm value of Tesla would be in the equity. In the case of 100 billion of outstanding debt, virtually all the firm value would be in the value of debt (and bankruptcy would be assured with probability near 100%).
