As for, "I am confused as to why it is necessary to go through this entire calculation." you have a provision that is available for an employee or a self-employed person, applies to the SE/LLC person as both the employer and the employee side of that relationship; and the evaluation for how much is "earned" is much simpler when the person is paid via payroll.

"I mistakenly reported the Employer matching contribution amount on Schedule C Pension/Profit sharing line instead of Schedule 1, Line 15 (See below description in BOLD)."

Is your person an S Corp sole shareholder-employee; a Sole Proprietorship; has earnings allocated from an LLC?

See if this IRS article, with an example, is helpful:

https://www.irs.gov/retirement-plans/one-participant-401k-plans