Its estimates, based on experience.
And certainly the US has experience in war over the past couple of decades.
Why would you think that applying the same metrics to a Russian war would be different?
If anything, these guys are basing their metrics on a US war. Do you think the Russian Army has demonstrated a more effective handling of the wounded, or provides better fighting conditions for their troops?
Some folks think in a war situation that the media should be handing out specific, detailed, and 100% accurate information. And they point that out as the reason to dismiss a report in its entirety. That opinion is kind of weak.
And the same people you trust have been LYING about war over that same time frame...have you read the Afghanistan Papers