Yes, I'm inclined to agree. I am very struck by Sietze de Vries' very clear structure. The third little piece he played in the clip is very reminiscent of Orgelbuechlein I think. Maybe I'm exaggerating, but I feel that with a performance like that, if he'd been competing for Kapellmeister jobs in the early 18th C, he'd have been up there with Telemann, Bach and Walther.
I'm also unsure about the French; they make remarkable organs and are remarkable organists and composers, but the last time I found a clip of a French improvisation, there were two registrants making unprompted changes in registration, which suggests careful rehearsal and a clearly agreed performance. I'm not sure at what point improvisation gives way to "playing a composition of my own from memory". Both are admirable skills.
I'm absolute rubbish at improvisation, and as a congregational member, I know I'm not alone! I agreed with David Briggs' statements about the need for structure and development. I agree that a successful improvisation should have the audience wondering if it's an improvisation at all. In fact I'd go further: I think a really successful improvisation has the audience/congregation merely listening and enjoying without being aware that it is an improvisation. I think probably David Briggs would guess quicker than me, because as a very experienced professional, it must be very rare for him to come across a composition of which he's unaware: something novel is quite likely to be improvisation. For me, I can only go on the quality of the music as I don't have the global knowledge of repertoire.
I'm afraid I disagree much more with the Westminster Cathedral organist's comments. I personally don't feel any particular privilege to be part of a creative experience when I hear improvisation, and as a listener I'm not more tolerant - it's just music. I was alarmed about his comments about using more dissonant musical language than he normally would in a composition. I'm sure in his hands he knows what he's doing, and the dissonances are intentional, but to an inexperienced organist his comments could be taken to mean the sort of improvisation of which I've been guilty: the sort where dissonances happen because I play what I think is going to be the right chord without being 100% aware of what it's going to sound like, and then only realise truly what it does sound like after I've played it - and it isn't what ought to have followed the previous note, and then we have to panic about where the "music" goes next - so things progress in a random direction from misdirected chord to misdirected chord until I accidentally lurch into a recognisable cadence and grind to a relieved halt. I've experienced these improvisations as an audience/congregation member, and meet them with a combination of pained sympathy with the organist who's having to paper over a liturgical crack; I try to look polite when all I want to do is scream and run out of the building with my fingers in my ears! The sad thing is, I'll probably never know when I've been hearing really good improvisations, having not recognised them as such. I do feel gratitude when I'm aware someone's improvising and it feels safe.
I'm sure that if people are going to improvise, then they should be taught. I never was. I'm also sure that most of us leap in at the deep end, trying to do too much too soon. We try to improvise in full multi-part harmony without first learning how to make up a single line tune, or add a slow bass line to a very simple melody. It's like listening to a pianist playing Chopin, and sitting down at the piano expecting to do the same without first learning basic piano skills.
Anyway, Sietze de Vries is now high in my list of heroes. He's just such a pleasure to hear.
(edit: sorry, that's a bit of an essay. Just sorting out my thoughts...)