One of the big arguments people used to defend the actions of the Israeli government and military, including justifications given by the Israeli government and military, was that they weren't attacking or targeting Palestinian citizens but were instead fighting against the terrorist group which also oppressed Palestinians - drawing a distinction between Palestinians and Hamas, arguing that they were only targeting a terrorist group. That's different from saying they're fighting a country's army. The conflict was presented as Israel vs Hamas, not Israel vs the Palestinian army (which doesn't exist).
The terms used to define the nature of a conflict do matter, so it is relevant that Hamas's armed wing is a paramilitary organisation with loyalty to Hamas rather than to the state. A national army has its responsibilities primarily to its sovereign state and citizens. The Hamas armed wing answers solely to Hamas (and people telling Hamas what to do).