Yes that is correct.
Also, in your first post, the taxpayers' penalty for 2018 should have been waived because the IRS adjusted the percentage from 90% down to 80%, so while they didn't meet the 100% of prior year exception, they did actually squeak by with 80% of the 2018's liability being paid in because of the $7,000 in withholding (8,742 * 80% = 6,994) .
ETA - also from your first post - if you know they will fall short and be exposed to the underpayment penalty for 2019 , the final payment must be in by 1/15/20, AND even doing that the taxpayer can still have a shortfall in any of the earlier quarters. If the exception to the penalty has been met though, then YES, it is possible to pay the remaining balance due by April 15th.