If that was the case, they would have done it for MK's fireworks show seeing as
@Magic Feather said that Harmonious is the most popular nighttime show currently. If the GSATs were that low, they wouldn't be doing a special, they'd be tweaking it based on guest feedback like they did with RoL, which is not something they are doing.
I think we need to just chalk this up to Disney doing something relatively cheap to give extra content to Disney+. As good as it looked and with all the performers there, this cost next to nothing compared to the average Disney+ offering (even some of the lesser things on the service) and it also did very well for Disney on social media with the live stream trending high on twitter all night.
I'll also mention that Disney+ was marketing this as a World Music Day event for the service.