An MD (Medical Doctorate) is like a master's degree. It's not like a bachelor's because many MD programs start out with or require a BSc, biology is a popular choice but a lot of STEM majors are possible.
But MD+PhD programs exist and those are definitely PhDs.
You are right that an MD is not a PhD, though. Notice how they don't call it a PhD.
In many countries, the degree you must obtain to qualify as an MD is indeed a bachelor’s degree, the “Bachelor of Medicine, Bachelor of Surgery” (often abbreviated MB ChB or MBBS).
I agree it's not a research degree though... But some master programs don't include writing a research thesis either.
"Professional degree" is juxtaposed with "research degree". So if you say it's a professional degree, you're basically agreeing it's not a research degree...
In my country at least MDs are not required to be researchers and the degree has no novelty requirements. They're required to be competent medical professionals.
There are countries where the base medical degree is the MBBS and MD is a graduate research doctorate. That's not what I'm talking about here.
I'm not talking about "difficulty", by the way. Just the differences between the degrees.