Yes, technically it should be just one carriageway (with change:lanes=* and turn restrictions as appropriate). However, mappers sometimes see this as pedantic, since crossing over the line is unlawful (even moreso than crossing over a double-yellow line in some situations).
Part of the problem is that change:lanes=* is being used to represent both single and double solid white lane dividers. Single solid white lane dividers are vastly more common, but technically they only discourage lane changes. For example, in the scene below, when turning right from this driveway onto westbound Waycross Road, it’s perfectly OK to cross the solid lane dividers to get into the left turn lane, but if you’re already on Waycross Road, you aren’t supposed to cross these lines just to change lanes.
By contrast, crossing a double white lane divider is always a traffic violation, but it’s tagged identically as change:lanes=no|not_left|…. The separate carriageway at least serves to emphasize this separation. However, in practice, no router would ever recommend crossing a lane change restriction along a straightaway like the one you found on I-405, so in my opinion, there isn’t much harm in combining the carriageways.
I was referring to the situations where there is unquestionably a physical separation, such as a Jersey barrier. This is also common in urban freeways that have collector-distributor lanes.