Assessment is a particular interest of mine, and I think that it is generally misunderstood and misused. You can't just attach a percentage from a survey like this to a letter grade as if that has some sort of meaning without any context. For example, it's not like, say, a math test, where 100% is an achievable outcome - you will never score 100% on a complicated customer satisfaction survey.
Grading is normally used to assess individual students in a particular context. Maybe you are diagnosing their current skill level (diagnostic). Maybe your are assessing their current progress (formative). Maybe you are assessing their mastery (summative). Here, we're not assessing individual achievement, we are assessing popular response to various proposals - basically emotional satisfaction among a huge population. Totally apples and oranges.
On top of that, percentages are generally thought poorly of as assessment measures, and in any event the association of particular percentages with particular numbers or letter grades is highly culturally dependent. For example, on summative assessments my IB school uses rubrics to identify specific skills in each subject, with students earning a final grade of between 1-7; everything but a 1 is a "pass." Unless you are writing your Extended Essay or taking Theory of Knowledge, both of which I teach, which do use letter grades, E-A, and a D or better is a pass (and a grade of D will earn you 75% as a university entrance equivalent at any North American university, a C gets you an 89, a B=94, and an A=100). Conversely, public schools in BC use a combination of rubrics, percentages, and letter grades, but they don't look anything like the numbers you've associated above. Here, anything from a P and up is a pass, and Ps begin at 50%, Cs at 55%, Bs at 70, and As at 85. Point being that stating that scoring 70% on any particular assessment is a C just doesn't track. At all.
You'll basically find as many different grading systems as there are assessment regimes. So the sentence
is completely meaningless.
My impression, from WotC's public statements, is that given the typical range of responses they consider 70% to be very good and 80% to be excellent. If you had to put that in terms familiar to a typical American student, that would equate to 70% being a B and 80% being an A. Sort of - it would be a hugely broad comparison and still not very meaningful. These are just completely different assessment tasks.
