First, if a bunch of rocket scientists want to keep UFOs a secret, they're going to come up with a better system than, "Hey, if you see a UFO, say Santa Claus over a non-secured frequency."
Secondly, anyone who has the vaguest notion of the context of the transmission knows what Lovell meant.
At the time of this transmission (Christmas Day, 1968), Apollo 8 was coming back around from the dark side of the moon for the last time. If they didn't make this burn and get out of orbit, well...
For all you Bowie fans, it would have been a whole lot like "Space Oddity."
It was incredibly stressful, make or break.
If they answered Mission Control when they were supposed to, we were going to the moon before 1970. If they didn't, Apollo 8 had become a tragic failure.
Here's an excerpt from the PBS show "Race to the Moon":
Chris Kraft: We lost the signal exactly at the right time, when they went behind the moon, and everybody, at that point, got up and started walking around in the room and I got on my intercom and said, "Look, you guys, do what you want to do but I'm going to sit here and I want to pray a little bit and I'd like to have it quiet here because this is one hell of a tense moment for me and for those guys in the spacecraft. So, for God's sake, be quiet for me."
Mission Control: Apollo 8, Houston, Apollo 8, Houston.
Susan Borman: And there was just dead silence. I mean you really could have heard a pin drop. No one was breathing. No one was moving and waiting to hear something. Because all you heard was Mission Control saying "Apollo 8". You know there was a one-way transmission. "Apollo 8, Apollo 8." Nothing, nothing, nothing, nothing, nothing.
Apollo 8: Houston, Apollo 8. Over.
Mission Control: Hello, Apollo 8. Loud and clear.
Apollo 8: Please be informed there is a Santa Claus.
Mission Control: That's affirmative. You're the best ones to know.