Per EOB, "the employer for IRC §404 purposes is the partnership, not the individual partner, so an extension on the individual partner's federal tax return has no effect on the deadline for making deductible contributions for the taxable year of the partnership, even for contributions made on the partner's behalf."
However, there is no citation provided and 404(a)(6) makes reference to "taxpayer" not "employer." Hmmm ...
Edited to Add: Rev. Ruling 81-114 makes reference to "the time prescribed for filing the employer’s income tax return" so the intent was to align the deadline to the deadline for filing the employer's tax return.