These are confirmed and identified casualties using 3rd party evidence. From the source you linked,

However, these figures represent only a partial account and do not reflect the full extent of the casualties. […] This week, CIA Director William Burns penned a column in Foreign Affairs estimating the total losses of the Russian army—killed and wounded—at 315,000. At first glance, this figure might seem significantly different from our own count, but in reality, it’s not, and we regard Burns’ estimate as close to the truth.

The reality is that we will not know for a long time the true numbers, but this 43k is the absolute minimum number of dead Russian combatants.