Many BMCs will tell you what hardware your server or other system has in it. This is a useful, even valuable function, but you shouldn't necessarily trust what your BMC is saying. We recently had a very vivid demonstration of that, where the BMC of a server confidently reported that the hardware had a number of inexpensive NVMe SSDs, a bunch of (expensive) RAM, and a pair of processors, but mysteriously the server wouldn't power on at all. When we opened up the server, we found no NVMe SSDs, no RAM, and especially no processors, which did rather explain the failure to power on.

Since the BMC in question offered us the option of downloading the raw SMBIOS information, my best guess is that the BMC gets its hardware inventory by having the main system's BIOS push the SMBIOS blob to it when the BIOS powers on or otherwise goes through a state change. The last time the system was powered on, it presumably had all of those NVMe SSDs, the RAM, and the processors, and now the BMC has that state latched until the next time the system can power up.

I was going to say that it made sense from a technical perspective too, but the more I think about it the less I'm completely convinced of that. I believe that some of the hardware bits in question report their status over I2C and other separate channels, not over the main system PCIe bus, so potentially they could be queried by the BMC through a direct BMC connection to appropriate I2C busses. It seems even NVMe devices can have separate I2C connections, so they could plausibly be inventoried separately. Whether this can be done with trickle power with the system powered off is another question. Perhaps what's going on is that the BMC can get hardware information independent of the system processors, but only when the system is powered up enough to supply power to everything, and that can't happen with no processors.

(This also shows vividly how a BMC can be a completely separate thing from the main system. There's no processors or RAM in the server, but the BMC is perfectly fine. It would be nice if the BMC could notice this and tell us, but I suppose this is a quite unusual situation that's not worth adding extra hardware or software for.)

Looking back, this isn't the only time I've seen this sort of BMC behavior. I've seen more than one type of server where some of the hardware sensors reported through IPMI over the management network were only available when the main system was powered on. In the case of sensors this was obvious because they reported no value when the main system was powered off. In the case of our hardware inventory, presumably the vendor decided that the value of the BMC reporting something when the main system was powered off outweighed the problem of it being potentially out of date.

PS: The interesting thing on top of all of this is that the BMC offered to let me configure the server's BIOS (through an entirely separate area of the BMC's web site, which switched to HTTP Basic Authentication). That's a neat trick when you have no processors, but maybe the BMC has software (and hardware) that can write to the NVRAM where the BIOS stores those parameters. It's also a bit disturbing when part of the BIOS parameters are processor dependent, but probably that's using the same BMC-cached information as the hardware inventory.