After the flood there was a 2nd irruption of them. So the mating with the "sons of God" happened again. And how did those giants produced after... the flood get wiped out, since the ones before the flood were wiped out by the flood? Simple, this is why God told the children of Israel to destroy them, for the giants dwelt in those nations of Canaan which God told Israel to literally wipe out, and leave alive nothing that 'breatheth'.
You make it sound like some escaped the pit through a volcanic eruption. They were not bound tight enough in chains of darkness?
Do you think any other genetic races need to be wiped off the face of the earth?
The sons of God were not angels. They were multiple humans created on the 6th day. The angels, morning stars, were created on the 4th day. Genesis 1:14-19
"And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: And let them be for lights in the firmament of the heaven to give light upon the earth: and it was so. And God made two great lights; the greater light to rule the day, and the lesser light to rule the night:
he made the stars also. And God set them in the firmament of the heaven to give light upon the earth, And to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good. And the evening and the morning were the fourth day."