VAT (Default surcharge)

Kaplan Revision Book FA2019: Knight Ltd., page 193

Task (Extract): Knight Ltd. was late in submitting the following VAT return and in paying the related VAT:
Return Period VAT due
3 months ended 31/12/2017 20,000
3 months ended 30/09/2019 30,000
3 months ended 30/6/2020 21,000
All of the company’s other VAT return have been submitted on time, and the related VAT liabilities have been püaid on time.

Answer (which I understand)
The late payment for the quarter ended 30/09/2019 —> a surcharge liability notice up to 30/9/2020.
The late payment for the quarter ended 30/06/2020 —> 2%*21,000 = 420 and surcharge period extended to 30/6/2021.

Answer (which I don’t understand, can you please explain this sentence below?):
The late submission for the quarter ended 31/12/2017 IS IRRELEVANT, as it was followed by the SUBMISSION of FOUR (?) consecutive VAT returns on time.