You are quite right, it was a mistake. I've corrected it now.
While it would be convenient in some ways to use just numbers, the percussion section can't be dealt with this way in any case. Apart from the fact that numbers won't tell you which percussion instruments are needed, percussion sections I've worked with usually decide on their own how many players they will use to cover the parts. I suppose in many cases, different orchestras will come up with the same number, but not always. I've never seen this section dealt with by numbers. The standard numbering system covers only woodwind and brass, and not the "extras" -- percussion, harp, keyboards, the odd mandolin part, etc.
Also, there are differing systems for dealing with doubling instruments, and even I (who regularly double within the clarinet family) sometimes can't remember how each system works, though I know the repertoire, so at least for my own section, I already know what's needed most of the time.
A numbering system has been discussed before, but never went anywhere, perhaps because it doesn't give a complete picture - if you have to look at a written out listing to get the number of percussion players and whether there is harp or keyboards, you might as well look at a written out list for woodwind/brass.
Another subsidiary issue where the written out list helps enormously is that for student (and some amateur) orchestras who not only need smaller orchestrations, but have horn players that don't read E flat horn, or clarinetists who don't have A clarinets, the written out list will tell them whether to skip a piece on the basis of availability of transposing instruments and/or transposition at sight.
I think it's a case of gaining some advantage, but at the loss of some other advantages.
By the way, mistakes will happen with any system, and it's not just our editors and librarians -- scores should give full instrumentation on the same page, and conductors/orchestra administrators, or anyone choosing repertoire shouldn't have to wade through to page 168 to find an instrument that wasn't listed on page 1, but it does happen. Anyone filling in instrumentation from a first page will most likely get a correct instrumentation, but not always . . .
Corrections to instrumentations are MOST welcome, and we will correct any mistakes people find.
Incidentally, I added a comment to the page about the common practice of using bass clarinet for 4 notes of the bassoon that diminuendos to pppppp. It is handed over from 1st clarinet anyway, so the timbres match better, and the dynamic is easier on bass clarinet.