It’s more that their knowing what an hour is would be impressive. Our selection of the hour as a measure of time is arbitrary outside of its specific context. It’s just 1/24th of our planet’s rotational period. We could just as easily split the day up into 10ths or 15ths or 7ths or whatever.
To broadcast a signal that’s exactly an hour long to a planet that uses the hour as a measure of time might potentially imply someone trying to reference our way of measuring time. A signal that repeats every 53.8 minutes is on a timer that isn’t specifically relevant to Earth in the same way an hour exactly would be.
That signal might be insignificant to us; but it may be their way of establishing a timescale.
The time may be derived from how long their planet takes to rotate…aka the length of one sub-unit of their day…aka 1/24th of their day.
