The word antīquus has a long vowel in the middle, but the proposition ante is short; indeed, if I am not mistaken (it has been a while since I read and wrote about Saturnian), both of its morae are short. If looking at only the word ante, we notice that its origin is the same as the first half of antīquus (*h₂énti), and the Greek is indeed with a short i (ἀντί: /an.tí/), as is also the Sanskrit (अन्ति (ánti)).
The vowel length of Classical Latin antīquus, according to what is stated at Wiktionary, goes back to Proto Italic. Now, I don’t know which Greek cognates (if any) there are, but Wiktionary lists अन्तिक (antika) for Sanskrit, again with the short vowel. My question thus, restated: How did the i in antīquus get lengthened, all the way back in its Proto Italic form *antīkʷos, when this does not seem to appear in its cognates, nor in Latins own ante¹?
¹ Though that could be iambic shortening and lowering, I presume.