Article IV of the Treaty states in part, “No acts or activities taking place while the present Treaty is in force shall constitute a basis for asserting, supporting or denying a claim to territorial sovereignty in Antarctica or create any rights of sovereignty in Antarctica.”

https://www.asoc.org/advocacy/antarctic-governance/overview-...

The countries claiming a slice of the pie aren't signatories to that treaty.

Actually, they are. The original 12 signatories of the Antarctic Treaty are the 7 countries that have claimed Antarctic territory, the US and the USSR (who both expressly reserved the right to claim Antarctic territory), and Belgium, Japan, and South Africa.

Which is to say, most of the people who cared enough to sign the treaty already had existing claims they didn't want repudiated by the treaty (which is why Article IV, in its entirety, also says basically "this doesn't affect any existing claims").