Look at the part in red they are not backdating it -
Electric car rates from 1 April 2025
Electric cars first registered on or after 1 April 2025: will pay the lowest rate of tax (first year rate), currently £10.
From the second year on, the standard rate kicks in and owners will need to pay £190 a year - the same rate petrol and diesel owners pay today (if they have a car first registered after 1 April 2017).
If the electric car costs £40,000 or more when new, the expensive car supplement will be added to the standard rate for five years.
As it adds to the standard rate, you will pay a supplementary rate during years two to six of ownership. Based on on today’s rates, that would mean electric car owners would pay an extra £410 on top of the standard rate of £190, to spend a total of £600 per year, for years two to six of ownership, before dropping back down to the standard rate from year seven of ownership and on.
Electric cars first registered between 1 April 2017 and 31 March 2025: will also pay £190 per year from April 2025.
The £40,000 expensive car supplement is not being backdated, so an electric car first registered in 2024, for example, will not have to pay the supplementary rate of £410 on top of the standard rate from April 2025.
https://www.which.co.uk/reviews/new-and-used-cars/article/car-tax-explained-aqnPf4D3c26Y