The Titans cameo in “Crisis” was just two recycled shots of its actors edited to make it look like the Anti-Monitor’s waves were killing them. The shots were reused from the series’ first episode of Season 2, with a completely different original context. None of the characters in Titans interacted with the Arrowverse’s characters in any meaningful way.
So it's recycled footage from Titans S02E01.
Also, Alan Ritchson who was Aquaman on Smallville and playing Hawk in Titans explained why he was not able to soot for Crisis:
“They did reach out to me. We tried to work it out. We were shooting and it was a very difficult schedule for us. We were shooting three episodes at once all the time. I would have had to leave the country and come back in to get a permit specifically for that show. I did want to participate. We tried to work it out. I’m happy that everybody is getting to come back. I’m especially happy for the fans. That’s why we do this. Those shows are like a bonanza for the fans. I wish I could have been a part of it.” sciencefiction.com
Hawk was one of the two Titans who appeared in crisis by reclcyled footage.