I don't think he is making the argument that anyone can/will do it, but that there is and will be plenty of mechanics willing and able to do the work and able to procure the parts, which he saying will not be the case with replacement battery packs on old cars.

There is some reason to believe this; stories like this [1] are going around, where EV owners have been told they need to replace the battery pack but, by the way, the part cannot be procured.

[1] https://www.wtsp.com/article/news/local/electric-car-battery...