Can someone ELI5? The Tweet and the "This will cause no issues" feels tongue-in-cheek and the gif suggests there will most likely be issues.
It will most certainly cause some issues.
I worked somewhere that sold a software appliance back in the day. This software appliance did a bunch of ETL and ran analytics processes. We had a leap second that caused all the ETL jobs to stop working. 400 some appliances in different data centers that we had some connectivity through a VPN. It was not a fun experience but it has always kept leap seconds and time problems in the front of my mind.
The first issue has already been found: https://twitter.com/uputronics/status/1418990637299519493
The issue is that only certain defined gps weeks use those bits for that purpose (when to actually apply the upcoming leap second), and it doesn't happen that often.
But anyone who assumed that an upcoming leap second message implies a non-zero value hasn't done their basic homework. It's a regularly recurring message, not only done when required.