Sure, either side could cancel. But Anthropic needs compute, and they found it in SpaceXAI. Why would they cancel the deal unless they don't need more compute or if they could get compute for less elsewhere (but where would that be realistically)?
Anthropic have deals in place to get another 5GW from Amazon, and 5GW from Google, just that it is not available yet. For comparison the total of Colossus #1 and #2 is 1GW