I'd like some clarification on what it means to "advertise your buy on GH." Does it boil down to, "If you post in the Group Buys subforum, you must post all future updates in that thread as well?" Or does advertising include making an IC thread or putting a link in your signature? For instance, if I open an Interest Check and at the end of the thread, lock it with a post saying "this buy will be run on this other platform, here's a link," am I obligated to post all updates on GH because I've used it as a platform to advertise my buy?
I think this is a very good rule in general that just requires a bit of clarification.
This rule is specific to threads in the Group Buys and Preorders subforum, it only applies to people who start threads in that section.
In the situation you propose, there would be no issue, since you've used the IC subforum for its intended purpose (checking interest, gathering feedback), and made the location change clear. What would be an issue is if you didn't lock the IC thread, and continued updating that thread once you were taking orders, or if you started a GB thread and abandoned it before the buy was complete. It's the second case in particular that this rule change is designed to prevent.
More generally, rules for Interest Checks are unchanged -- once you're ready to start accepting orders (or taking money for any reason), the IC thread should be locked, with an indication of where users can go for future discussion. If you as the GB runner have decided at that time that you don't want to run the GB through GH, and would rather just do it on Massdrop, or DT, or somewhere else, that's fine, just link people over there when you lock the thread. But if you're running the GB on some other platform, and also maintaining a GB thread here, you need to continue to update that thread until the buy is complete and everyone has received their order.