you can't compare ecash to phoenix. phoenix is an implementation of a protocol that aims to fix or minimize issues with the protocol it implements. ecash is a concept with multiple protocols, as I understand it. im assuming the NUTs for cashu are not used in fedimint. the difference is the protocols based on chaumian ecash by nature of implementing ecash have to have a centralized mint for creating tokens. lightning does not inherently require this centralized authority, even though in practice the centralization can make transactions simpler, as seen with phoenix.