The Israeli government itself says it can’t confirm beheadings. Media and foreign governments, sure, they might just not have access, but Israel has the actual dead, or at least direct access to who could confirm their state.
This all seems to have started with a quote from an IDF guy that they were “butchered”, which seems like something that can telephone along to become “beheaded”, but may have just meant killed without being able to fight back. Which is already plenty bad.