WebMD is correct; taking levodopa with MAOIs runs the risk of a hypertensive crisis and levodopa should not be taken for at least 2 weeks after MAOIs; the risk of taking RIMAs (specifically moclobemide) with levodopa is one of increased side effects which include anorexia, nausea, vomiting, agitation, postural hypotension, tachycardia, arrythmias, hypomania, psychosis, depression, headache, flushing, gastrointestinal bleeding, itching, rashes, abnormal involuntary movements.
AFAIK, l-tyrosine taken with MAOIs renders it more likely to be a vasopressor ie cause an elevation in BP.
The thing to recognise about such advice is that some people may not suffer such adverse effects, but there is a significant likelihood of having such a response that it can only be considered wise to avoid such combinations.