Tom,
"Unnecessary force" key. There was certainly a thud on the landing, but I have seen numerous side lifts with authoritive mat returns and nobody ever thought of calling a slam. I have seen a ton of college wrestling over a lot of years and since Bo did not pile drive the kid, nor did he drop him on his head or cause any harm I disagree. It sounded like a possible slam, but visually it to me did not look like a slam.
Unnecessary force is a big component of determining what is and what is not a slam.
While I won't argue that you've seen a lot of college wrestling, most of what you stated was a fan's perspective, and not what a ref uses to determine a slam.
Slams are not limited to "pile driving," or "dropping a wrestler on his head," or "causing any harm." In fact, the last criteria you listed, "causing any harm," is not even a factor. A ref can't wait to see whether a wrestler is injured or harmed before ruling that a slam took place.
As the rule that I quoted indicates, a slam can happen from any of the three positions in folk -- top, bottom, or neutral. The rule uses the term "unnecessary force" and "excessive force." The determination of what constitutes that is left to the referee.
Any time a wrestler is lifted off the mat, a ref is looking to see how their opponent returns them to the mat. Many refs will even state things like, "careful," or "return him with care," or things like that, when a wrestler is in the air, particularly if they are elevated to a high level, or up there for some time.
Other things a ref will consider include (but are not limited to): is the defensive wrestler's arm trapped (meaning they have no chance to break or cushion their fall); does the offensive wrestler land on top of the defensive wrestler (so that their weight comes down on top of the defensive wrestler), how hard does the defensive wrestler land (sound can be included in this evaluation, though it's also how the move looks).
More than anything that's in the rule, the ref's experience of seeing 10s of 1,000s of moves where a defensive wrestler is brought back to the mat are what make help them to make the determination whether or not to call a slam.
When Bo brought Brunson to the mat, my hands went right to my head -- the signal for a slam. I'm pretty confident that 99% of wrestling refs would have called it a slam. It really was a pretty easy call.