Source 1 is inaccurate. There is no direct add back of passive losses to compute AGI. However, there might be is an effect on allowable passive losses that have an AGI-based phase-out.
Go to the authoritative source, IRS Pub 590-A:
https://www.irs.gov/publications/p590a#en_US_2018_publink1000230985
TurboTax's implementation of the calculation of MAGI for the purpose of a Roth IRA contribution is based on the IRS instructions.
