Ah, but that article is about OD&D, and in the designer note articles around the development of AD&D one of the stated design goals was to slow down progression a bit, in order to increase the playable lifespan of the game. Gygax (or maybe Lawrence Schick, I don't remember which one actually wrote the specific article I'm thinking of)
Can you remember where that article was?
claimed that the most active groups were able to "exhaust" the possibilities of OD&D in about a year of play, and that it was hoped that with AD&D it would take more like 3-4 years for that to happen (which is, of course, funny when nowadays there are campaigns that have been going nonstop for 20+ years). So 6th level after a year of weekly play is probably about right for AD&D.
I feel the big difference about AD&D comes at the upper levels (7+) compared to D&D, with much better monsters, spells, magic items, etc. Once you hit 8th or so in oD&D, there wasn't really that much more to conquer, but 7-12 were fleshed out a bunch in AD&D.
For oD&D+Greyhawk, I feel the rate of XP gain is on a parity with AD&D. Of course, role-playing/narrative-heavy games will tend to be slower, but that isn't exactly how the game was expected to be played!

Of course, a lot of my assumptions about level gain are based on the campaigns I played... which were using official AD&D modules. I wonder how much my DM fiddled things?
If you assume 1 level per six sessions (I think that's a standard placed in one of the Mentzer boxed sets of the BECM game), that means after a year of play you should be about 8-9th level.
Cheers!