There are many interesting sources on the destruction of Carthage itself (I haven't really read much on the take of modern historians), and indeed there is a reference by the sources on a seventeen day fire.
Lucius Florius, who wrote a brief sketch of Roman history some 250 years after the event (based on Livy's work) says:
Personally I cannot find that particular statement on the other sources, and Florus is known for being a bit loose with details and for putting rhetoric first.
In any case, the destruction of the city is more than well attested. Even if the fires were partly caused by the Carthaginians themselves, the Senate clearly decreed that nothing was to stand where Carthage used to be. Not only did the city suffer systematic fires and famine across the siege, but the slaves sold apparently weren't that many, most Carthagians either dying of famine, killing themselves, dying in prison or just being executed. They were literally wiped out the face of the earth instead of assimilated, and while speaking of genocide on this context has little meaning, you can certainly describe it as such under a modern perspective.
Other interesting bits from Roman and Greek historians showcase this:
Cassius Dio states:
Polybius, Scipio Aemilianus's friend writes:
Appian's account is the most gripping one for me (as it provides a good degree of detail on the gradual destruction of the city as the Romans advance during the siege), and among other things you have:
Diodorus Siculus compares it to the suffering the Greeks face around the same time over the burning and sack of Corinth:
Finally,
Velleius Paterculus:
