The etymological relationship with δέος is correct. The analysis as a compound containing λίαν is not. The principle of this explanation is the same as that in Plato, Cratylus 415c (see δεῖ / λίαν). The difference between both is that Plato starts from a sequence verb + adverb, whereas this one starts from a sequence noun + adverb: that cannot match a corresponding phrase since adverbs never modify nouns, but the internal syntax of compounds was not a question for Greek etymologists.