Sticks were heavier then than they are today. Johnson was bigger and stronger than most other players; perhaps he was the only one strong enough to maintain dexterity with a longer, heavier stick. Pure speculation, but so is assuming he must have used it because he was a weak skater. I play defense in rec hockey, and use the longest stick I can find at the sports store. I'm also an above-average skater relative to the other players in the league. I'm not trying to mask a skating weakness by using a long stick, it's just a personal preference. Maybe it was for Moose too.
To reiterate your point, there is no correlation between using shorter/longer sticks and skating ability. Defensemen often use longer sticks for valid reason. Players often experiment with shorter/longer sticks and various lies. It's a common theme that choice of stick length and lie is essentially defined by your playing style, not by your skating ability. There is a general consensus if one wields a stick which is considerably too long, it may be detrimental to many facets of their game.
I think it's prudent to say there isn't anyone alive who's qualified to comment on Moose Johnson's skating ability. The best one can do is regurgitate what they discover in old newspaper clippings. If we suggest he was an inferior skater because he wielded a long stick, we may as well infer his IQ as the highest in the league because he wore a helmet. It's fascinating to note that Wikipedia suggests he was one of the few skaters who could skate backwards and simultaneously engage the play during his time. That's definitely an interesting tidbit when considering the general skill of the skaters during that era. It apparently also suggests he was a better overall skater than most in his heyday.
I had a look at the provided picture of Moose Johnson with the Portland Rosebuds, and my initial thought was that his stick didn't appear to be remarkably long, maybe up to his chin on skates. I did observe the players to the far right are wielding shorter sticks, relative to modern standards. See picture below:
I did some quick digging into the newspaper clippings, and discovered an article which states, “his stick from handle to heel is five feet one inch.” This measures his stick at 61 inches. See picture below:
Using a modern-day example, a current Bauer senior retail stick measures at 60 inches from the heel to handle. If Moose Johnson measured at 5 feet, 11 inches, this would have placed his stick at about chin height while on skates. I’d use Nick Lidstrom’s stick length as a comparable example and a maximum estimation of his stick length, while on skates. See picture below:
Johnson’s equipment styling in combination with his unique (at the time) skating ability may lend credence to his 'clever' moniker. It appears that he was a little ahead of his time in both regards. It also suggests that ehhedler was a little clever to include him in his list, as well.