What they forget to mention is that the site wasn’t merely for garbage; it was a place for burning. The place wasn’t only a dump, it was a crematorium. The bodies of dead criminals were thrown and consumed there.
And you’ll find this elsewhere. For example, Hagner (WBC) states (117):
The name Gehenna is from the Aramaic words גֵּי חִנָּם, gê ḥinnām, for the “valley of Hinnom” (cf. Josh 15:8; 18:16), a despised place to the southwest of Jerusalem where at one time human sacrifices were offered to the god Molech (cf. 2 Kgs 23:10; Jer 7:31) and where in later times the city’s refuse was burned. The constant burning there made the valley a particularly suitable metaphor for eternal punishment
Or in the NET it reads:
This was the valley along the south side of Jerusalem. In OT times it was used for human sacrifices to the pagan god Molech (cf. Jer 7:31; 19:5–6; 32:35), and it came to be used as a place where human excrement and rubbish were disposed of and burned.
And the Analytical Lexicon of the New Testament’s article on Gehenna states that it is
…literally valley of Hinnom, a ravine south of Jerusalem where fires were kept burning to consume the dead bodies of animals, criminals, and refuse; figuratively in the Gospels and James for hell, a fiery place of eternal punishment for the ungodly dead
The problem is that this seems to lack historical evidence. From a footnote (Bibliotheca Sacra 155 (Jan.-Mar. 1998) 324-37) conveniently found here:
The traditional explanation that a burning rubbish heap in the Valley of Hinnom
south of Jerusalem gave rise to the idea of a fiery Gehenna of judgment is attributed to
Rabbi David Kimhi’s commentary on Psalm 27:13 (ca. A.D. 1200). He maintained that
in this loathsome valley fires were kept burning perpetually to consume the filth and
cadavers thrown into it. However, Strack and Billerbeck state that there is neither
archeological nor literary evidence in support of this claim, in either the earlier intertestamental or the later rabbinic sources (Hermann L. Strack and Paul Billerbeck, Kommentar zum Neuen Testament aus Talmud and Midrasch, 5 vols. [Munich: Beck,1922-56], 4:2:1030).
I quickly consulted more of my sources for example Davies & Allison (ICC, p515):
The standard view, namely, that the valley was where the city’s garbage was incinerated and that the constantly rising smoke and smell of corruption conjured up the fiery torments of the damned, is without ancient support, although it could be correct.
…but I could find only a few that committed to saying the same thing. They almost all say that it is a place of continually burning garbage, some (like the dictionary article above) add the cadavers.
Now, this wasn’t a fundamental point to my post (that the place had burning garbage and corpses) since I based most of my conclusions based on what Christ was saying about Gehenna. So all my previous points, I think, still stand: Gehenna is a bad place that is to be shunned. If this corpse and garbage burning is non-historical then I actually think it makes Jesus’ usage all the harder to refute—but we don’t know, so I retract it for honesty’s sake. It sounds pretty good, it might very well be true, but as of right now, based on my scanning, we only have evidence from some twelve hundred years later.