It's debatable. Fidelity is paraphrasing section 408(d)(4) of the tax code:
(4)Contributions returned before due date of return
Paragraph (1) does not apply to the distribution of any contribution paid during a taxable year to an individual retirement account or for an individual retirement annuity if—
(A)such distribution is received on or before the day prescribed by law (including extensions of time) for filing such individual’s return for such taxable year,
However, section 219(f)(3) creates the legal fiction that a contribution for the preceding year was made on December 31 of that preceding year:
(3)Time when contributions deemed made
For purposes of this section, a taxpayer shall be deemed to have made a contribution to an individual retirement plan on the last day of the preceding taxable year if the contribution is made on account of such taxable year and is made not later than the time prescribed by law for filing the return for such taxable year (not including extensions thereof).
Because section 219(f)(3) says, "For the purposes of this section," it's not clear if that carries over to section 408(d)(4) because the it relates to the same contribution.
