Democritus was going with the notion that, if you kept cutting a substance in half (as Dimitra Triantafyllidou explains the verb), an atom is where you got to when you couldn’t split it any more.
tmētos and a-tomos are both adjectives derived from different variants of temnō “cut, split”. There is no adjective *tomos “cuttable” corresponding to a-tomos “un-cuttable”; but there is no meaning difference that I can tell between the two.
The -able is only implicit in a-tomos: it’s quite literally “uncut”, as a permanent state of affairs, ergo “uncuttable”.
There is no necessary -able notion in tmētos: it actually is defined in LSJ as “cut, shaped by cutting”. LSJ defines some –tos adjectives as “X-ed” (dartos “flayed”, gyristos “rounded, curved”), some as “able to be X-ed” (dēlētos “able to be shown”, detos “that may be bound”), and some as both (dektos “accepted, acceptable”, and Manolis Fanourgakis’ word, diairetos “divided, divisible”).
But yeah, if you want an opposite to atomos in Greek, tmētos is the closest you can get; and in the right context, tmētos could have been interpreted as “cuttable” (i.e. divisible) rather than “cut”.
diaireō means “take apart, cleave in twain, divide”; so it could have been used instead of temnō for what Democritus had in mind (adiairetos, “undivided, indivisible”). But while diaireō in classical times could still refer to chopping things, it was more commonly used about dividing things for sharing, distributing. atomos is explicitly about chopping things.