Technically, not even that. The question was "what is the source for this information?" and what I said was "conjecture on the part of the poster - plenty of reason why the relationship between those killed would have warranted a visit for reasons other than a planning meeting; they were literal family".
But, no, I did not say they were not terrorists. They were members of Hamas and PIJ, FWIW. Source: Hamas and PIJ. What their roles were, whether they were combattants, etc.: no bloody idea! And it also does not matter!
But even if: hors de combat is a thing. In other words: even in war, you cannot simply walk into an enemy field hospital and massacre all the patients. That is still a war crime. And for good reason! These rules were established in the aftermath of the horrors of two world wars. Clearly, we are not doing very well at remembering how we once came to the conclusion that even war needed rules!
Edit to add: Israel, of all countries, ought to be bloody happy about the Geneva Conventions, Paris Accords, and similar components of so-called ius in bello [law at war]: with (near) universal conscription, any failure to distinguish between "actively engaged in combat" vs. "currently unarmed" vs. "injured" vs. [you name it], basically any Israeli within a certain age bracket could be reasonably assumed to be a combattant and, therefore, a legitimate target. As someone who has met plenty of Israelis in my life and has been friends with some: I do not want that for them! (Not even for that horrible former boss of mine, who was an active IDF reservist! I might have wanted to strangle him multiple times a day for work-related reasons - but: no, you do not get to assassinate him sitting at his desk in the office ... much as I would - again, for work reasons - have found it hard to be particularly sad for him on a personal level.) That is just, literally, not how "the rules" work. And we should all be glad! A world in which "might is right" (which we de facto have - but at least we pretend) is a dark and scary place for literally every single human on the planet. Any single one of us might come across "the guy whose gun was bigger than my own" on any given day!