I wonder why he didn't just count down in minutes if it was to give them a heads up for landing. I very much doubt most crew will know what altitude relates to in time to go anyway, not to mention the variables involved.
Only time I've seen altitudes mentioned is due to vacuum system problems with the toilets and they must he closed off before getting below a certain altitude. 18,000 rings a bell during descent. Perhaps a warning to shut the toilets off? I've never had to do it but the is a procedure for doing so.