I think he's implying "until yesterday".
I've used GoDaddy for 10 years or so, and this is the first DNS outage I can remember.
That's an impressive record, imo.
(Personally I've largely switched off GoDaddy to Namecheap now, but not because of uptime, rather because I grew tired of the sucky GoDaddy website and sucky APIs)
Basically they're guaranteeing 99.999% uptime across some interval and you get some kind of compensation if/when they fail to meet it.
I just misread their statement. They probably meant the up-time before this incident.
"Throughout our history, we have provided 99.999% uptime in our DNS infrastructure. This is the level our customers expect from us and the level we expect of ourselves. We have let our customers down and we know it."