You can get it after the ride if you upload it to Strava or TrainingPeaks. Speed is a byproduct of all other data and should not be a primary means of judging fitness.
No where in the original post did they say anything about using average speed as a means to judge fitness.
You shouldn’t really need a 3rd party account to see your ride data. Pretty sure I’ve seen Zwift say that post ride data analysis is coming. I also like to see the average speed of my rides particularly outdoor rides, less so Zwift rides.
Yes, Zwift has said that an improved post ride data analysis is coming.
As far as average speed goes why would you need it? Are you trying to compare rides based on average speed? Your average speed can be affected by terrain, type of bike used in game and/or drafting. You can ride the same course, using the same watts and have a different average speed because you choose to use a different bike in game or where in a group ride. As I said, speed (and average speed) is a by product of other factors and is has no real meaning.
Averages of any of the metrics is meaningless and that includes watts, heart rate and cadence.
Also, this has been requested a lot, make sure you vote up the existing requests so Zwift knows that the users want: https://support.zwift.com/hc/en-us/search?utf8=%E2%9C%93&query=Average+Speed&commit=Search