As far as I can tell from IRS Rev. Proc. 2019-19, only excess elective deferrals can be corrected by distribution. Excess allocations (employer contributions) are instead to be held in a separate account in the plan to be allocated to participants in subsequent years and the excess is subject to a 10% excess contribution penalty on Form 5330 each year that any excess remains until the separate account is fully consumed.
Since there was a distribution, this apparently was a corrective distribution of excess deferrals. Chapter 4 of IRS Pub 560 describes the taxation of a distribution of excess deferrals made after April 15 where it states that this is subject to double taxation.
Rev Proc 2019-19: https://www.irs.gov/pub/irs-drop/rp-19-19.pdf
2020 IRS Pub 560 is presently only available in draft form: https://www.irs.gov/pub/irs-dft/p560--dft.pdf
