It's not accurate to say it was "entirely unsubstantiated". Other news sources besides the NY Post (most recently the Daily Mail) have claimed it was authentic, and Biden himself hasn't disputed its authenticity.

Regardless of the laptop though, there's clearly a double standard in what Twitter will block with or without proof. The Russian bounty program that was widely reported by the Washington Post and others during the election cycle was unsubstantiated, but no effort was made to block it.