They are indeed found in many products, but the key is the exact nature of the specific substances. Do they inhibit MAO-A, MAO-B, or both? Are they irreversible or reversible inhibitors? Do they do other things like cause neurotoxicity? The Harmala family has dozens of compounds, some good for you (for example MAO-B inhibitors indeed prevent PD, because MAO-B inhibitors are used to treat it) and some bad, like some beta-carbolines that are indeed neurotoxic.

Again, the total amounts are what matters. It’s much easier to take in too much MAOIs from tobacco and coffee than by eating meat.

Something seeming to protect against one specific disorder does not mean that it protects against all diseases.