The IRS clearly stated in Notice 2020-18, Section III, that "Federal estimated income tax payments (including payments of 3tax on self-employment income) due on April 15, 2020" are within scope and that "the period beginning on April 15, 2020, and ending on July 15, 2020, will be disregarded in the calculation of any interest, penalty, or addition to tax for failure... to pay the Federal income taxes postponed by this notice."

This means Q1 ES-tax underpayment penalty will not kick in until Jul 16 although the penalty for Q2 will start on Jun 16 as usual.  Just imagine (1) treating Q2 as if it were Q1 (and vice versa), (2) changing the trigger date for Q1 penalty from April 16 to Jun 16, and (3) replacing the trigger date of Jun 16 for Q2 with Jul 16.