They're comparing to Haiku, not Opus. Haiku is currently at 4.5.

Even if it were Opus, comparing to a version number makes for an interesting snapshot of time comparison: if you knew how a model performed at whatever time in was in vogue, you can say "well, it looks like Model X is about 6 months/1 year/etc. behind the frontier SOTA" - which is exactly the discussion that happens in the open-weight/local LLM space. (interesting, MAI-Code-1-Flash does not appear to be such an open-weight model, following the western trend of locking models up)

Latest Haiku (smallest Anthropic Model) is version 4.5, they haven't released a new version, hence the comparison to that.