I mean.... hard drives are made of f*cking metal man.
Most metal is nearly infinitely recyclable. Once it's been extracted and processed initially, it has a fairly low carbon intensity. Silicon, on the other hand, is notoriously difficult to recycle and is extremely energy and carbon intensive in manufacture. You can't just take the same lump of silicon and recycle it back into a new lump of newer silicon -- at least in the same way you can aluminum housings and glass platters.
That said, I agree with your general premise. I don't buy that HDDs are better enviromentally.
I grant you, the manufacturing angle of that is ~probably~ minor. SSDs don't use a massive amount of silicon in the first place.
The power is a real concern - a drive that's on 24x7, efficiency really starts to add up. You have a lot of factors there - HDDs don't vary too much with load (the platters always spin, the drive arm doesn't take a lot of power), but SSDs do take a hit under load. SSDs can idle in the milliwatts, but HDDs have to keep that platter spinning -- once you get it up inertia helps, but you are still in the watts, not milliwatts. Now, HDDs can "sleep" and spin down, but they incur a massive seek penalty to spin back up, and a big power hit to get the platters back up to speed. SSDs win the power part hands down, but the amount they win by varies widely based on the work load and how much you can let the devices idle versus sleep.
And yeah, once you bring capacity into the equation, then the massive density of the latest hard drives will crush SSDs, if you are looking at Watts per Byte Stored. I don't think you can look at it that way, you really need to look at watts per byte transferred (either read or write) - because you get constrained by the transfer speed of the interface. The storage density itself is irrelevant really, unless you are just looking at the cost to park data and have it available - not actually access it. Which isn't really a good metric, because that doesn't do anyone any good and you could get something like tape or iced storage to look like a freakin' rock star -- zero energy requirement past the initial write and huge density on what amounts to just plastic film (or glass optical media, if you really wanted to get environmental about it). Once you take the fact that HDDS transfer slower, and their density doesn't really do them any good ... yeah, I don't know how those numbers would look exactly, but I suspect SSDs would win out on that -- vastly faster with generally lower power requirements.