There are Arrow 3 and THAAD defence systems against the Houthi threat, but yes, it failed and airlines bailed. DL making a decision to return means they conducted a security assessment and decided to return. UA is on record as not wanting to return prematurely earlier this year only to have to leave again. But DL forced their hand and UA returned in March, only to leave earlier this month.

Competition is a massive factor on a lucrative route, and UA isn't going to let DL have TLV to themselves. I'd be very surprised if UA doesn't return in the first half of June, barring further risks to TLV. Had DL not returned, the overall assessment would certainly be different.

But UA giving different cancellation dates to the media and on Jetstream is interesting and seems indicative of internal debate on when to resume.