Ah, and here we get into which figures are right. Why would "The Government" know more than the NG?
And, obviously, your answer would be "And why would the NG know more than the government".
I don't know which one is the authoritative source, but I would (naturally, because it substantiates my point) tend to believe the people who run the thing more than "The Government", whoever they are... :D
Also, you assume the generating capacity will remain static (which it doesn't have to). Even if all cars sold in 2030 are EVs, there will remain ICEs in circulation for a long time after, so the transition will be slow enough that generating capacity can be brought online to more than supply the increased demand.
Ideally nuclear generating capacity, IMHO, but sadly I suspect I'm more likely to win the lottery two weeks running than THAT happening...