Comment by ehsankia

It's indeed very confusing, it's either

1. It's available to Pro in Gemini app, but NOT antigravity (for coding)

2. It's a phased rollout and Ultra got it first, Pro is coming later

The latter blog post is specifically "Argon in Antigravity" versus the other blog post is about Argon in general, so seems like #1 is more likely.