The issue has to do with the need for NTP servers to tell connected clients about the extra second.
If you ask for the time before and then after midnight, you'll just get a normal answer both times and think your internal clock had slipped by one second.
If you ask at exactly the right moment, you'll get a response which some clients will not expect and may not interpret correctly.
If the Instant only makes an NTP call at boot, and then never checks again to adjust the time, then there's nothing to worry about expect the time will be off by one-second until the next reboot.
If the Instant happes to boot at midnight on the fated day and gets the one unexpected tick, then it may do unexpected things.