I'd have to go with failure. The gear horn was going off the whole
time, and the lack of reaction at touchdown lends me to lean towards
"they knew it was coming".

Don't be so sure. I've had both students and rental-checkouts continue
down final with the gear horn blaring, completely oblivious to it. It
boggles my mind, but I've seen it several times. It isn't until I ask,
"what does that horn mean?" that there's a dawning.

Scary.

I also presumed that it was a mechanical failure. However, the description
at the bottom of the page states that "...the pilot thought his gear was
down. ..." It also invites additional information, if known.