You do realize that the purpose of hi, including bohi, is in no way related to tachikaze, right?
Traditionally NO. However the purpose of bohi on THOSE swords is to create tachi kaze. You're not going to be thrusting an alluminum blade into a man anytime soon to warrant the need for an air pocket to make the sword easier to withdraw.
In fact tachikaze is the purpose behind bohi on almost all modern iaito. People these days use it as a TOOL to help their technique. The person was asking about buying iaito. Because these days most styles practice / judge their technique by their tachikaze, I advised him that this wouldn't be a great sword to practice with.
Why did you even bring up the "purpose" of bo hi? It's not even relevent to the question that was asked.