I have never read Matthew 11.18 to mean that John ate absolutely no food and drank absolutely no water, but in James A. Kelhoffer's The Diet of John the Baptist (pp. 11-12) he suggests that this passage makes "the super-human claim that John survived without eating or drinking anything at all." And that this led to the inference, "that the Baptist was sustained by his mastery over, or the affliction of, an other-wordy being (δαιμόνιον ἔχει)." A parallel claim in Luke 7.33 mentions bread and wine specifically as the things which John did not eat. I've interpreted Matthew 11.18 to mean the same thing, essentially, but in shorthand. Maybe I've been wrongly reading Matthew through Luke though? Maybe Matthew is making the claim that some accused John of being sustained by demonic power because he went without food and drink in the wilderness (which, if true, may shed some light on Jesus' temptation in the wilderness).