The following is certainly worth noting, though strangle the commentary goes on to dismiss the reference as something “not to be believed literally,” without good explanation.  Regardless, with believing it literally the following is well suited to consider:

“Of much more importance is the unquestionable evidence of the Apostle’s belief in the pre-existence of Christ. He does not say, ‘And the rock is Christ,’ which might mean no more than, ‘And the rock is a type of Christ,’ but, ‘And the rock was Christ.’ In Gal. 4:24, 25 he uses the present tense, Hagar and Sarah ‘are two covenants,’ i.e. represent them, are typical of them. Similarly, in the interpretation of parables (Matt. 13:19–23, 37–38) we have ‘is’ throughout. The ἦν implies that Christ was the source of the water which saved the Israelites from perishing of thirst; there was a real Presence of Christ in the element which revived their bodies and strengthened their faith. The comment of Herveius, Sic solet loqui Scriptura, res significantes tanqam illas quae significantur appellans, is true, but inadequate; it overlooks the difference between ἐστι and ἦν. We have an approach to this in Wisd. 11:4, where the Israelites are represented as calling on the Divine Wisdom in their thirst, and it is Wisdom which grants the water. Philo (Quod deterius potiori, p. 176) speaks of the Divine Wisdom as a solid rock which gives imperishable sustenance to those who desired it; and he then goes on to identify the rock with the manna. The pre-existence of Christ is implied in ἐπτώχευσεν (2 Cor. 8:9), in ἐξαπέστειλεν ὁ Θεὸς τὸν υἱὸν αὐτοῦ (Gal. 4:4), and in ὁ Θεὸς τὸν ἑαντοῦ υἱὸν πέμψας (Rom. 8:3).”

Robertson, Archibald ; Plummer, Alfred: A Critical and Exegetical Commentary on the First Epistle of St. Paul to the Corinthians. New York : C. Scribner’s Sons, 1911, S. 201