Perhaps so. I was intending to err on the side of safety. You said "find a way to gauge it by direct measurement."
Direct measurement of maximum HR involves performing all-out strenuous exercise while continuously monitoring heart rate.
When I said "if you haven't been in those ranges for a while, best to check with your MD to be safe," I meant that a middle-aged couch potato probably shouldn't go for Max HR without clearance. Almost anytime an increased exercise regimen is recommended, it comes with the advice to check with your personal care provider.
This is not overly simple; it's common sense. There is a lot of undiagnosed obesity, diabetes, heart disease in the general population.
This doesn't necessarily apply to healthy individuals with a solid fitness base, but it doesn't hurt to be careful.
See Jim Fixx and Edmund Burke for a couple relevant examples.