I’d say those guides are wrong.
Racial bonuses don’t tend to be super-important in general; as long as you don’t literally have a penalty to an important score, it’s probably fine enough. What are some things a druid wants from his race?
A bonus feat. Well, yes, dwarves don’t get this and it’s amazing. Shame to miss out. But this also applies to almost-everything that isn’t human. Even beyond that, druids need exactly one feat: Natural Spell. After that, everything else is gravy, and unlike literally every other class in the game, it is optimal (barring planar shepherd, which you should) to take 20 levels of druid without multiclassing or taking a prestige class, so druids don’t have to burn feats on meeting prerequisites.
A bonus to Wisdom. Dwarves lack this, but it’s also extremely rare in the first place. The only options are decidedly weird, not to mention frequently banned: anthropomorphic bat (blatantly overpowered), buomman (can’t speak), jermalaine (Tiny-sized and ridiculous ability scores), and lesser aasimar (clearly overpowered, though not as much as anthrobat).
A bonus to Constitution. Hey, dwarves have got this!
Some way to communicate while in Wild Shape. Options here are rather limited, but a few races exist that do it.
So yeah, humans are probably better druids than dwarves. Which is nearly meaningless, as humans are probably better than almost anything else at almost everything. For the dwarves, the Constitution bonus is good and the Charisma penalty is minor. The bonuses on bull rush, trip, and various saving throws are quite nice as extras. The weapon familiarity goes to waste, and the armored movement thing is a bit harder to use, but those are minor anyway. The low land speed doesn’t matter much because it’ll be replaced when in Wild Shape.