It can also involve deviations in strategy based on the count, particularly with borderline hands.

Your understanding is correct, more or less[0], but there are two parts to strategy: an inexpert counter is likely to be distracted, and to make errors from perfect play. They’re also likely to lose the count, and make errors in bet sizing. The net of those is worse than a non-counter playing perfectly, whose edge is slightly negative but who still stands a decent chance of making money on a given day.

But note that’s a reason for casinos not to overtly discourage counting; they’ll still happily ban a player who is apparently counting well rather than roll the dice on whether they’re counting “well enough”.

[0] Sibling points out that counters will make specific deviations from “naive” perfect play depending on the count, but that’s to push earnings up a bit on an already positive edge. There’s also the element of camouflage, where a really strong counter might deviate in ways that don’t hurt their earnings but make their play look less “counter-y”.

Ya but simple mistakes can turn +ev to -ev, and if you miscalculate and scale up on -ev, then that can wipe out your +ev for hours