I like the concept of the Teacher TwitterBrain, a lot:
I think there are likely two components as to why this ended up working the way it did:
1. You have created a community as your audience. The people reading your blog or following you on twitter, genuinely care about you and your work. They want to see good ideas, but they also see you as a human being and not some faceless teacher imposing assignments on kids. This community is a support structure just as much as it is a learning one. I think that calling out the fact that you have probably cultivated this community over years of your work is important.
2. You shared it in a collaborative way, specifically requesting feedback. You could have easily shared this assignment as view-only. You could have easily made it into a PDF and then sent it out. But, you didn’t. This was a collaborative process of sharing with multiple entry points (throughout the assignment folks could comment on different things). This means that you are much more likely to get the kinds of outcomes you are looking for. So, while I agree with your last paragraph that “sharing is caring.” It really matters how you share too. If you share something static, you will not see the power of the Teacher TwitterBrain. If you share something open and honest, then you very likely will.