The Steampunk tag does not come from just the engines fuel source. There is a lot more to both Steampunk and Diesel Punk than the fuel type.
For example Mad Max can be considered Diesel Punk, and Mad Mac has a very different style and setting than GoIo.
GoIo has a lot of influence from Victorian science fiction writers as well, many of which are now classed as "steampunk" by genre.
The tag refers to the style, which based on design, back story (especially backstory!) and fashion sense, GoIo is much more Steampunk with some dieselpunk influence.
Also "80%" is an inaccurate figure for ships powered by diesel.
So although GoIo has Dieselpunk influences, it is far from the genre of diesel punk.
Watch water world, mad max, mad max 2, the new mad max (Do not watch Thunder dome!) and research waste landers to see more diesel punk and compare to GoIo