Yes, this is true. It's not a huge factor. Keep in mind that the worst 1% of the days of the year are a serious strain on the electrical grid. They often require very inefficient and expensive peak plants to spin up to help ensure the grid doesn't fail. Not only are those peaker plants inefficient when running, they are quite expensive even when not running.

Additionally even on those top 1% days (3.5 days a year) there's likely only a few hours where the load is near max.

So if say 1M (2% or so of all cars) cars in a country like Germany would contribute say 25% of their capacity just 3-4 days a year it could make a significant difference and make the power grid more reliable and allow the least efficient peaker plants to be decommissioned.

Even over a decade of use, electric car owners aren't going to notice 25% of their battery being used 3-4 times a year.