To answer your specific question--and not opine on the conclusions of the WaPo reporter whether the details of President Trump's civil matter factually warrant such a claim-- the Federal Election Commission has a FAQ page that contains answers on penalties. It reads:
What penalties can the FEC impose for violations of the law?
Most violations of the Federal Election Campaign Act (FECA) result in civil penalties--fines arrived at through a conciliation process. Knowing and willful violations of certain FECA provisions can lead to imprisonment. The FEC has exclusive civil enforcement authority, and may refer criminal violations to the U.S. Department of Justice. For additional information see our page describing the complaint process. Note that sentencing guidelines for criminal violations of the law are set by the U.S. Sentencing Commission.
If this was a filing error, it most certainly would fall under the Administrative Fines Program Criminal violations are reserved for matters like embezzlement.