I was enthusing about the LUMEN Jam over on r/RPGdesign and a solid question came up. Zireael07 asked, "Nitpick: there is an SRD, but it doesn't say how it's licensed. Is it Open Game Content? Open Game License? Creative Commons?"
IANAL; From my reading of the landing page and the SRD itself, this appears to be a simple 'gentleman's agreement.' Essentially, 'give me credit and I won't take legal action.' I'm fine with that, but this might be a barrier to some people, therefore some form of license might be in order, if only to make other potential designers more comfortable with using LUMEN.