It's definitely weasel words. Open Source has a definition. The source may be publicly viewable, but that isn't "open". It's still closed, and it being viewable is actively harmful, as it'll lead to copyright violations sneaking into other projects.
Also I think that the assertion that it's "actively harmful" is wildly hyperbolic. Sentry isn't going to be out there copyright trolling people and you know it; they're trying to protect themselves from a very specific category of IP infringement. And there is still much potential upside to it's openness that you're willfully ignoring.