That’s not the case. Using the recently hosted D1 and WC2, people have been connecting to Battlenet and playing on both WC2 and D1.
The issue is, the version of D1 that Bliz is selling has a bug where it installs the old legacy servers (East, West, Europe, Asia). Those can’t be connected to with the version Bliz is selling.
Thus, a fix is needed to remove those gateways and add the “Global” gateway, which is the modern, intended gateway.
Using the original CD version, even patched to 1.09, will put people on the legacy gateways, essentially splitting the online playerbase.