Of course 538 adds the tied version and not the Biden +3 version as well.
They almost always add results with leaners when they are available.
If you're doing a model and average though, why not add both just for more data?
They do that for polls with alternate turnout models, but their polling average - not sure about their model - has always worked this way as opposed to de-emphasising the preferences of leaners, which is what adding both results would do (some polls only release the final result with leaners anyway, so at least this ensures consistency).
