My understanding is that the employee elective deferral limit is across all employers. Using the "Maximize Contribution to Individual 401(k)" option, it suggests that I can elect to defer another 23k in addition to an employer contribution (that matches what I get from other calculators).
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).