So, the question is "Does or doesn't Steam allow developers to share their sales figures?"
The answer, in our experience, is yes...mostly. It's a little complicated.
So, when we first started working with Steam, we explicitly asked them whether we could share our sales data, and they said, yes, it's your data, do what you want with it.
Eventually, after this post on our linux sale results I got contacted by our steam contact, and they said they wanted to clarify/revise the policy. They were very polite about it (they never asked me to take the article down or anything), and they explained that there's kind of a thin line to walk here.
Basically, a developer can share their own sales data if they want to, but it's against Steam's policy to reveal that information itself.
Now, here's where it gets tricky. If I share MY sales data down to the very last penny, under certain circumstances smart people on the internet can reverse-engineer other people's data. For example, if Defender's Quest were to show up as top seller #7 one day, and I post what my exact sales where for that day, people can reasonably guess that whoever was #8 made less than us and #6 made more, and now by allowing me to post that super-precise data, Steam has put itself in an awkward position with those other developers, who might have wanted that data kept private.