Deductible self-employed contributions that appear on (2018) Schedule 1 line 28, including a SEP contribution, do reduce your AGI and therefore reduce your MAGI for the purpose of determining the maximum Roth IRA contribution that you are eligible to make. The amount on line 28 and the deductible portion of self-employment taxes on line 27 are not added back to AGI when calculating MAGI.
We SuperUsers have complained many times to the moderators here that TurboTax's MAGI FAQ is very misleading. Ignore it.
The reference to IRA contributions being added back is an add-back of any deduction for regular individual IRA contributions on Schedule 1 line 32, not line 28. The amount on line 27 for the deductible portion of SE taxes does not get added back. I'm not aware of any MAGI that requires the add-back of the amount on line 27.
Additionally, though, while the amount on line 28 does not count toward your regular individual IRA contribution limit, both the amount on line 28 and the amount on line 27 must be subtracted from net profit from self-employment in determining the amount of self-employment compensation that remains to support a Roth IRA contribution.
For most people, MAGI for the purpose of a Roth IRA contribution would just be AGI, plus any regular personal IRA contribution on line 32, minus any addition to AGI resulting from a Roth conversion or taxable Roth rollover from a qualified retirement plan to a Roth IRA (but not an In-plan Roth Rollover). The other add-backs are much less common for anyone approaching the Roth IRA contribution phase-out range. For specific details, refer to Worksheet 2-1 in IRS Pub 590-A, which is what TurboTax implements.
