Learn French: Il pleut à tel point d'inonder la rue. - It rains to such a point of flooding the street.

Both express a consequence meaning 'to the point of', but adding tel (such) in à tel point de puts extra emphasis on the intensity of the rain. It translates closer to 'to such an extent' or 'so much so'.

Why do we use the infinitive inonder here instead of a conjugated verb?

You use à tel point de followed by an infinitive when the subject of both actions is logically the same. Here, the weather (the impersonal 'it' in il pleut) is also what is doing the flooding (inonder). If you wanted a different subject for the second action, you would use à tel point que plus a conjugated verb, like Il pleut à tel point que la rue est inondée (It is raining so much that the street is flooded).

The English translation says 'It rains', but wouldn't 'It is raining' make more sense here?

Yes, 'It is raining' sounds much more natural in English for an action happening right now. French does not have a separate present continuous tense, so the simple present Il pleut covers both 'It rains' (in general) and 'It is raining' (currently).

Can inonder be used figuratively, just like the English word 'flood'?