Well there's really no way to prevent it and guild hopping is still possible, it just requires a bit more coordination - there's nothing stopping you from kicking a few members to make space for some (or just one, depending on the raid) heavy-hitters to come in and help you through a tough raid. We just had a sister guild do a Heroic Sith Raid that way 🙂
Besides, the notes and text description imply that this should now be possible, and it isn't. If the fix in question was not meant for this scenario then go out and say so and update the notes so it's more clear. And if it for this scenario and it's still not working then go out and say so as well 🙂
