I'm not going to debate the question of whether or not Israel is committing genocide wrt Gaza, but this argument is just dumb. He's basically saying it's not genocide because they haven't killed enough people; since they could have killed all of the Palestinians in Gaza but they didn't, it can't be genocide. The term is loaded, of course, and has consequences, which is why there is such an intense debate, but according to the UN:
In the present Convention, genocide means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
Killing members of the group;
Causing serious bodily or mental harm to members of the group;
Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
Imposing measures intended to prevent births within the group;
Forcibly transferring children of the group to another group.
https://www.un.org/en/genocide-prevention/definition
Under that definition, at least, you don't have to kill everyone in a targeted group, or even most of them, if killing or harming members of the group is done with the intent to destroy the group, in whole or in part. It doesn't set a percentage. I'll let others debate whether genocide according to international law is occurring in Gaza, but I don't think the fact that there are still some Gazans alive and walking around rules it out as the above-cited author, a right-wing pastor, suggests.