The distribution from the 401(k) would only be excluded from the MAGI for determining your eligibility for a Roth IRA contribution if the 401(k) distribution was rolled over, either to a another pre-tax retirement account (which would keep it from being part of your AGI in the first place) or to a Roth IRA (in which case it's subtracted from your AGI in determining your MAGI for this purpose).
See line 2 of Worksheet 2-1 for IRS Pub 590-A: <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p590a.pdf">https://www.irs.gov/pub/irs-pdf/p590a.pdf</a>
