Capital allowance would be time apportioned if the account period is less than 12 months. The period of account here is 12 months (6 April 2018 – 5 April 2019). As such Kaplan’s calculation is okay.
N/B It does not matter what time in the year the asset (car) was bought, so long as the account period is a complete 12 month period you allow the full business use.