That would be incorrect. Corporations are allowed SEP Contribution deduction for the year calculated (2019) or year cash actually contributed (2020), as long as it was contributed by the due date of tax return including extensions. See IRC Section 404(a)(6) "Time when contributions deemed made".
Also see IRS Issue Snapshot and "Timing of Deductible" Examples here: https://www.irs.gov/retirement-plans/issue-snapshot-deductibility-of-employer-contributions-to-a-401...
To answer the above question, the deduction would be allowed on the 2019 Corporation tax return.
