From documentation:

According to my tests such interval schedules checks at 9:00:30, 9:01:30, 9:02:30, ..., 9:59:30, 10:00:30, 10:01:30, ..., 10:59:30, ...

Comments in the code shed some light on what happens in this case:

Empty filter matches all valid values if the filter level is less than interval filter level. For example if interval filter level is minutes - m30, then hour filter matches all hours. If the filter level is greater than interval filter level, then filter matches only 0 value. For example if interval filter level is minutes - m30, then seconds filter matches the 0th second.

So the problem is not only that example or it's description is incorrect, but that scheduling interval documentation is incomplete.