I don't think this necessarily targets the same audience as Sonic All Stars. Sonic games have been catering to kids for a while now, which is why they did so well on consoles like Wii. This KS didn't get funded in 40 minutes because kids are busting out their credit cards. It's because people who were 10-16 in the late 90s when the platformers on the N64 that this is making no bones about drawing influence from were released are now grown adults with disposable income. This is the audience of gamers that are probably now all splintered across various consoles. This is why it was really smart of them to not separate consoles by different stretch goals like a lot of KS's do. Just put it out on everything and make it clear it's coming to everything.
No I don't agree at all. They're very much a similar audience. Yes it's true, this backing is mainly done by people longing for the old days of Banjo Kazooie, but kickstarters are often a very small percentage of the final sales (even in other cases of games where they target specific enthusiasts, e.g. Shovel Knight.) The KS of this game will be more split toward the other consoles, I agree, but ultimately this will cater primarily to kids and those with a penchant for mascot-based games that are often found on Nintendo systems.
Do I think it would be better to wall it off to Nintendo consoles only? Absolutely not. That's silly. Do I think this will do better numbers on Wii U that the other consoles? I'd say it's likely.