Instead of all the guessing, I asked IB about this.
Even if you're on tiered, your directed API orders are on the fixed commission schedule, meaning you'll pay 0.0075. That's what I understood beforehand also by simply looking at commissions of executions.
"Alessandro B: Fixed - US API Directed Orders: USD 0.0075 per share (see: https://www.interactivebrokers.com/en/index.php?f=1590&p=stocks1)
Please note that directed API orders cannot use the Tiered fee structure. Smart-routed API orders can use either the Tiered or Fixed structure, see Note: https://www.interactivebrokers.com/en/index.php?f=1590&p=stocks2
a*********: I've found the page
Alessandro B: The above note is at the bottom of the page
a*********: hmm, so while I'm on tiered, API directed is charged based on fixed?
Alessandro B: Exactly"