They can sail there, but above 500 passengers they can't land at all. Those voyages are cruise-by only, so you'd see Antarctica from the rail and never set foot on it. If the point of the trip is standing on the continent, that rules them out completely, and it's the single most important thing to check before booking a big-ship Antarctic sailing.
The bands that matter:
| Passengers | What you get |
|---|---|
| Up to 200 | Up to two landing sites a day, everyone ashore |
| 201 to 499 | Landings, usually one site a day, groups rotating |
| 500 plus | No landings. Cruise past only |
Between 201 and 499 you do land, you just get less time and less flexibility, because the operator is working a bigger rotation through the same 100-person cap.
Big-ship Antarctic sailings are usually cheaper per day and they're a legitimate product for someone who wants to see the place without the expedition commitment. Just buy them knowing what they are.