Whilst I agree that in this particular case there wasn't much of a threat to troops (even if the Houthis knew beforehand I highly doubt they have any capabilities to take down US jets), the main national security issue is what this means, that high ranking officials are exchanging top secret information on insecure chanels. If a foreign adversary were to hack their phones, or if like this case, someone who shouldn't have been added was added and the info was leaked, this could lead to much more sensitive and dangerous information getting to the wrong hands.