Thanks, rjwb. I did some further investigation on my Dell XPS8900. I finally noticed that HWiNFO has tiny fan icons next to the sensor values that are denominated in rpm. I found these sensor values under the heading "DELL EC: DELL XPS 8900". When highlighted, this heading gives the following explanation: "This sensor provides data from the Embedded Controller via SMI interface. . ."
I tested the sensor values. After starting my computer with the case open, I gently placed my finger on the smooth center surface of each fan to slow them down briefly, and the sensors showed new minimum values of roughly 500 rpm, about half the normal values. Interestingly, there was a new (higher) maximum value recorded also, which I assume was a rebound from the slow-down. The fans then settled down to normal speed. This suggests to me that, under normal operation, the fan's speed is still being limited electrically, not by friction.
I am so impressed at the great information provided by the motherboard and HWiNFO!