A lot of C. Tolkien's feelings and thoughts are quite understandable. However, especially when I look at the open-mindedness Tolkien himself seemed to have for the adaptions of his books (and some of his hopes that others might take over where he left of), C. Tolkien seemed to give himself the role of some sort of inquisitor, borderlining to be insulting and ignorant with some statements, one of which you have quoted. And all this while he gave himself quite some freedom as well with the Silmarillion and The Children of Hurin (for which he is also criticized by some of the hyper-purists, I will not judge on that, just mention it to show how easily ones harsh policy can turn against oneself).
I always understood, that JRR Tolkien wanted to create a would-be-mythology for England. A mythology others read, told of, worked with, etc. That is exactly what has happened - as with the mythologies of old. Sure, it happens in modern ways, but it has happened. Arda and its history has come as close to a "real" mythology as probably any modern collection of stories and novels will ever come (with "real" I mean the mythologies of the ancient world). And the movies did add quite a bit to this legacy (let alone multiplied the financial outcome for everybody involved with any works of Tolkien).
But when I read C. Tolkien's comments on the movies, I did not feel he was treating his father's work as a mythology, but more like a book written in stone - with the act of changing or adapting something (especially for a 21st century audience) being close to herecy.
But to close the cycle, I do agree with what you wrote. If one scratches some unkind comments by C. Tolkien he has acted rather wisely by not publicly saying much about these works and we (or I) shouldn't put words into his mouth or thoughts into his brain. He certainly has given us Tolkien lovers a lot to be thankful for. What I have written about inquisitors, herecy and books written in stone is actually something much more applicable to many self-proclaimed Tolkien-hyper-purists than his honourable son.