1. It is not the tank. It's the maturity of the tank. A mature tank can process ammonia, nitrite, nitrate before it became a problem.
2. Stable parameter is necessary. The salinity can reach 1.030 and nitrate can be at 50ppm and the BTA is fine as long as it is NOT a spike.
3. Getting a healthy nems. Generally speaking nems don't do well when ship. BTA is more forgiving. But imagine a BTA shipped to a whole seller, and then it is shipped to the retailer. Additionally, many members buy bleached nems. Those aren't healthy to begin with.
4. Your experience and knowledge of anemone matters. When people buy a fish, they look up compatibility, tank size requirement, feeding requirements and so on. So many people buy an anemone and just flopped it in and then ask why did it moved?
The 6 months is a recommendation. If you know what you are doing, you can add it whenever. You can setup a 10g with just water and put in an anemone and it can be just fine like that forever; assuming you have the lighting requirement, filtration and flow. I'm sure I forgot something.
Happy anemone reefing. A few of my babies.