Months old thread, but since you were left without an answer:
Yes, they could have switched to Trixie months ago. Changing to stable now that Trixie is released will let them somewhat automatically upgrade to Debian’s next stable version when it is released. That is why they used testing instead of Trixie, so that they would automatically get the new testing version on release.
But this is generally not recommended. It is recommended to only use the codenames to keep more control over when you upgrade.
Novice question: Couldn’t you have switched to Trixie months ago and also don’t have to change to Stable? I thought that would give the same result.
Months old thread, but since you were left without an answer:
Yes, they could have switched to Trixie months ago. Changing to stable now that Trixie is released will let them somewhat automatically upgrade to Debian’s next stable version when it is released. That is why they used testing instead of Trixie, so that they would automatically get the new testing version on release.
But this is generally not recommended. It is recommended to only use the codenames to keep more control over when you upgrade.