By the way, I've decided to break my public note-taking into separate threads to make it less unwieldy, with CWs for headings, tagged #Bible to make it easier to mute, since I'm aware that the subject matter can be pretty sensitive for some. Apologies if earlier formats were disruptive.

The editorial priorities are good to keep in mind when reading Biblical texts. It seems clear that the person or (more likely) group who put Genesis into its present form was working with a set of relatively diverse materials, likely pulled from several traditions with already permeable borders. For example, we know that Babylonian mythology had its own flood narrative, but the Genesis editors appear to have been working with multiple Israelite traditions, possibly associated with multiple tribes contained in the traditional Israelite polity. So there seems to be some effort, OTOH, to piece together different not-quite-related stories into a single, unified narrative — that's one thing accomplished by the rather tedious genealogies connecting, say, Adam to Noah — and that has to be balanced with an effort to draw in traditions that will tie Israelites together by including the traditions that are important to various cultural subgroups.
#Bible

For example, a common motif in traditional myths and legends is the invention of cultural institutions that might otherwise seem timeless. And from the Eden narrative through the Noah narrative, agriculture seems to be invented multiple times, by multiple personalities. Mostly, the editors don't seem too interested in the invention of agriculture, though — they're just drawing in different traditions for their own purposes, and those different invention narratives are embedded in the final text because they were integral to the traditions the editors had at hand.
#Bible

Reading the #Bible, you sometimes catch glimpses of traditions that weren't quite central to what the editors were trying to achieve, but that were common enough to the audiences they were writing for that they were worth keeping in, without much explanation. For example: "The Nephilim were on the earth in those days…" Oh. Okay. The Nephilim. And who were they exactly? The Bible doesn't go into it much, and the Nephilim don't really do much of anything in the narrative. The editors were compiling for an audience that would have already known that tradition, and the reference serves to ground the text in the familiar, but not much else. And I think a lot of the difficulties that crop up in trying to understand the Bible can be addressed by starting with the premise that the compilers were trying to produce a text that people would embrace, and thus had to work within the confines of traditions their audience already endorsed. Sometimes, you'll even see them attempting to gloss a tradition that doesn't really fit the direction of a chapter, and it's a good bet that the gloss is there because they couldn't just outright ignore the tradition, for fear of losing their audience.

Back to the Nephilim and "the heroes that were of old" for a moment: I think what's going on here is that the compilers are dealing with an inconvenient tradition, first by acknowledging it, then by placing it prior to the flood, so that it can be dispensed with. They're saying, "Yes, of course, there were demigods at one point, just as you've always heard, but they were killed before everything we're going to talk about here." Why? Because the editors are focused on a fundamental different theology, one premised on covenants with the Lord. They want to foreclose the possibility that the messiahs who crop up in the rest of their narrative are demigods, and frame them, rather, as upholders of the covenant. The fact that they have to deal with the Nephilim first testifies to the popularity of the tradition that they're expunging via the flood. And, indeed, before the end of the chapter, the Lord promises to establish a covenant with Noah.