Unless I'm missing it, the page they're referring to (https://openai.com/index/introducing-gpt-5-3-codex-spark/) never claims Spark is 15x faster.

It looks like it only appears in the snippet the Google result shows, presumably taken from the meta tags. It's possible an earlier draft claimed a 15x speed boost and they forgot to remove the claim from the tags.

I think they modified the page. If you search for GPT-5.3-Codex-Spark, Google still has it indexed with 15x. Searching: GPT-5.3-Codex-Spark + "15x" will show all the downstream sites that picked up the claim.