There are lots of injuries deemed 'ankle fractures' that are much less serious than breaking your tibia and fibula. Jack Roslovic returned 6 weeks and 4 days after fracturing his ankle last season. I agree with you that most ankle fractures take much longer than 6 weeks to return to action. 6 weeks is very much optimistic given what we know and my guess is that we are talking about 8+ weeks.
But we don't know any of the details about his specific injury and there are scenarios where it was minor enough that he "only" has to stay off it for 4 weeks and then can begin strengthening the surrounding muscles while putting weight on it. We don't know whether it requires a surgery. 6 weeks in a boot/cast is pretty much the minimum following a surgery, but a small fracture that doesn't require surgery doesn't carry the same timelines.
My gut tells me that we probably don't see him until after Christmas, but the Blues have been cagey about using the term "will be re-evaluated in X time" for the last few years. Given how many times we've seen Blues returning right at the initial timeline, it appears that the Blues use that term to refer to the time they believe the player could be cleared to play.
