I'm going to assume that they're referencing this study published in Lancet which estimates "186,000 or even more deaths could be attributable to the current conflict in Gaza". This is from combining the known direct deaths with estimates for indirect deaths such as from the ongoing famine.

The shortcomings of this estimate have to do with the lack of comprehensive review of the damage. The IDF won't exactly pause for several months while such a review is conducted, and the Gaza Health Ministry's ongoing count of the direct dead is likely far below the actual number due to the difficulty of counting every single body.