If you read the article they've just done simple maths by subtracting the number of known named terrorist deaths from the IDF database, from the Hamas total death figure.
The article says itself
The intelligence sources explained that the total number of militants killed is likely higher than the number recorded in the internal database, since it does not include Hamas or PIJ operatives who were killed but could not be identified by name, Gazans who took part in fighting but were not officially members of Hamas or PIJ, nor political figures in Hamas such as mayors and government ministers whom Israel also considers legitimate targets (in violation of international law).
The article is incorrect in asserting targeting political figures is in violation of IL, as depending on the circumstances they can be as follows:
When a minister becomes a legitimate target:
Direct Participation in Hostilities (DPH):
If a minister is directly involved in military operations or decision-making that constitutes participation in hostilities, they lose their civilian status and protection for the duration of their involvement.
Effective Contribution to Military Action:
A minister's office or their actions might be considered a legitimate target if they "make an effective contribution to military action" and their destruction, capture, or neutralization would offer a definite military advantage.
Also civilians participating in hostilities:
Civilians are protected against direct attack unless, and for such time, as they directly participate in hostilities. Parties to an armed conflict must take all feasible precautions in determining whether a person is a civilian and, if that is the case, whether he or she is directly participating in hostilities.
In case of doubt, the person in question must be presumed to be a civilian and protected against direct attack. To protect civilians, combatants – and anyone directly participating in hostilities – must distinguish themselves from civilians in all military operations by wearing identifiable insignia and carrying arms openly.
Looks like the assorted terrorist groups in Gaza have been exempted from that bold bit.
Hamas is also said to have recruited around 30,000 ‘youths’ (who will be recorded as children in the casualty rate) during the war, which should make pro-Palestinians not vociferously publicly calling for Hamas to surrender sick to their stomach because it’s pretty obvious Hamas has been strengthened to continue by the global support they have had for their cause, which includes those who remove them from the equation when focusing on demonising Israel including the OP of this thread.
Do they support child soldiers, as young as 12, being used as combatants?
The argument that it’s better for civilians, including pregnant women and children, to die as human sacrifices for Hamas than evacuate them to a safe country because they may not be able to go back to their already devastated homeland for a while is fucked up enough, but to know child soldiers are being used by terrorists in a war as cannon fodder and STILL not be marching and protesting for Hamas to surrender is beyond my comprehension!
https://www.timesofisrael.com/hamas-said-to-recruit-30000-gaza-youths-into-its-military-wing/amp/
Has there ever been a war where one side has been expected to be able to name all the dead combatants of the opposing side, in an ongoing war no less, anyway?