I wonder if this is the same as modern Greek, where "if" can also mean "when". I.e. "When I invade, I will turn you out". "If".
A more literal translation would be "upon", which can have both meanings. "Upon arrival" can both mean "if he comes" or "when he comes".
Looks like the original said "ἂν ἐμβάλω εἰς τὴν Λακωνικήν, ἀναστάτους ὑμᾶς ποιήσω", which is the word for "if", but I don't know if the meaning of "when" was there in ancient Greek too.
https://en.wikipedia.org/wiki/Laconic_phrase