I'd like to point out that you can't co-opt material from 5e under the OGL because that license doesn't cover 5e. 5e has no licensing agreements (outside those closed licenses announced for Kobold Press and Trapdoor Technologies).
However, rules mechanics cannot be protected with copyright. Only the specific presentation of rules can be. This legal loophole is the reason for the existence of OSR as Basic/1st/2nd Ed were not part of the OGL license either.
What's going to be interesting is the specific presentation of these books. They'll have to skirt a fine line to maintain compatibility while not violating Wizards of the Coast trade dress. That said, while OGL covers nothing from 5e, Necromancer is free to put anything THEY create under OGL.
I think it's a very bold and potentially risky move on Necromancer's part, depending on how WotC reacts. They might have a case on the tagline "5th Edition Rules, 1st Edition Style", but that's not clearcut. Nothing else in the Kickstarter seems to be in violation, but like I said, it depends on the text itself and how it's presented. Depending on how WotC reacts, this may end poorly in terms of relationships with the company itself for those involved regardless of any legal complications. But the rewards may be worth it, beating other companies to market with material for a new edition has always done well for third party companies. The biggest I remember was when Sword and Sorcery Studios got their monster book on the market before WotC got out the official Monster Manual back with 3rd Edition's release.