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I'd like to know how they ruled out dust/absorption as the explanation for the dimmer-than-expected light. And without lots of confirmatory observations, how can they infer that ALL distances to ALL galaxies, and hence the Hubble constant, is wrong? Moreover, M33 seems too close to use as an indicator of the Hubble constant; local motion can easily swamp it, as is the case for Andromeda, which is at a comparable distance.

Or am I missing something here?

I'd like to know how they ruled out dust/absorption as the explanation for the dimmer-than-expected light.

I'd guess that it's pretty much factored into such observations.

And without lots of confirmatory observations, how can they infer that ALL distances to ALL galaxies, and hence the Hubble constant, is wrong?

The article says they're going to do more.

Moreover, M33 seems too close to use as an indicator of the Hubble constant; local motion can easily swamp it, as is the case for Andromeda, which is at a comparable distance.

My very fast (and thus worthless) research suggests that using eclipsing binaries, you get a solid reading on mass, thus an excellent clue as to what the absolute brightness should be. It gets a bit shaky from there, as your first question indicates. I don't see any indication that local motion affects anything. Maybe redshift, but that's not involved here. It's an independent method.

Or am I missing something here?

It's likely that I am. We need RadioAstronomer, but he's out of town.

I remain unconvinced. We're dealing with a sample of one here. If they had gone through and hit all of the Cepheid variables in the galaxy, and then double checked them with this and perhaps one other distance indicator, then they might be on to something really interesting. This is merely a curiosity until more data is taken. The Hubble Distance Scale was done with far more checks and balances than this, so for them to overstate their case like this is a bit over the top, but to the alumni, I'm sure it sounds impressive. Those Buckeye alums are easily impressed (Go Blue!).

I'd like to know how they ruled out dust/absorption as the explanation for the dimmer-than-expected light.

Since dust doesn't absorb the same percentage of light at all wavelengths, it can be normalized out by comparing different bands. This is trickier than it sounds, because the density of material will be different at different distances (read: redshifts).

And without lots of confirmatory observations, how can they infer that ALL distances to ALL galaxies, and hence the Hubble constant, is wrong?

That's because of the cosmological distance ladder. They use parallax to measure the temperature-brightness curve of the Hertzprung-Russel main sequence, the Hertzprung-Russell main sequence to calibrate nearby Cepheid variables, distant Cepheids to measure the distance to type-1a supernovae, type-1a supernovae to measure the distance to significantly redshifted objects (there's your Hubble constant), and redshift to measure farther out. If the calibration of one of the early rungs is significantly off, then it throws all of the others off.

Suppose you're looking at an actual ladder stretching away from you. If you know the distance to the first rung to around 10%, and you want to know the distance to the eleventh rung, it doesn't help much to know that the second rung is 11.00003 +/- 0.00002 times farther away than the first; you still don't know its distance to better than 10%.

For myself, I'm skeptical of this claim. We know the Hubble constant indirectly from measurements that are independent of the cosmological distance ladder, and I don't think there's 15% uncertainty there. (I could be wrong about that, though.)