It's pretty much confirmed by OpenAI here [1].

> We generally treat GPT-5.5’s safety results as strong proxies for GPT-5.5 Pro, which is the same underlying model using a setting that makes use of parallel test time compute.

And Gemini also provides something similar. Gemini Deep Think models are pretty much the same thing [2]. As to why no other company uses this, I don't really know. Maybe compute constraints?

[1] https://deploymentsafety.openai.com/gpt-5-5

[2] https://deepmind.google/models/gemini/deep-think/