Why does Turbotax show that I can defer 23k as an employee in a self employed 401k plan, even though I maxed a 401k via my W-2 job (box 12 D and AA)?

TurboTax is incapable of limiting self-employed elective deferrals when you have made such deferrals at another employer, so you cannot use the Maximize function for contributions to a self-employed 401(k).  Because you maxed out elective deferrals at the other employer, you are only permitted make the employer contribution to the self-employed 401(k).  The calculation of the employer portion is the same as the calculation for a SEP-IRA contribution, so you can just use the Maximize function for a SEP-IRA contribution even though the employer contribution is to the self-employed 401(k).