IMO, the SDK is not a competing product. It would allow HST to provide a better Z-wave interface, spend less time on debugging/troubleshooting the Z-wave drivers, and concentrate more on other requested features. I don't see HST banning the Girder plugin, yet it could be considered a competing product since it supports home automation as well.
So, while I don't agree with the decision, it is their forum, they do pay for the software/hosting, so they can do as they please.
Most HS users would NOT drop Homeseer in favor of some SDK (and they spent the money on HS already anyways), and I can't imagine that any potential customer would NOT buy Homeseer because of a single thread pointing to the SDK announcement.