It's worth noting that Gravit.io is a JavaScript rewrite of Stagestack project [1] which was an attempt to create commercial clone of Macromedia FreeHand from scratch [2]. Stagestack project failed due to lack of funding from FreeHand community.
1) Joel was talking on the context of a software business -- if you have working code it can be detrimental to business to do a rewrite from scratch, it will take you years, you will leave stuff off, piss off your customers, lose ground to the competition etc. It's not like Gravit.io has that kind of "business reasons" at this moment (this is their initial version offering), so his reasons do not apply to it.
2) Gravit.io is a port of a project to another language. This is not a rewrite -- much less a "rewrite from scratch". Again, Joel's reasons do not apply to the situation.