Not that weird I think? Americans are a plurality and probably majority of HN commenters, and UTC is a universal time zone (hence the "U"). American astronomers would describe observations in UTC (with the American time format), regardless of the timezone the observation took place in, yet would still communicate between themselves using the American date format. (In contrast, it would indeed be weird if the European time format with the colon was intermixed with American date format, e.g., 1130 UTC on 3/4 for 11:30am on March 4th.)
That said, HN is sufficiently international that mentioning the date format to avoid confusion is good.
I guess amateur astronomers might use something else besides UTC or some form of Julian date to label their data, and the professionals might well put whatever date format they well choose in their directory names.
But looking at the Keck open data archive or the time conversion utility NASA hosts I found little evidence they frequently use anything else than standard iso dates.
It would make sense that Keck open data would use the international ISO standard, no? Further, NASA changed (in 1990) to the metric system, but this doesn't really change how two American astronomers would communicate with each other on a day-to-day basis.
It'd be pretty weird for american Astronomers to use a strange date format for their observations, but not strange if they used them when talking about their next seminar.
That said, HN is sufficiently international that mentioning the date format to avoid confusion is good.