I think they're suggesting that 1 year certificates are still at the point where people can just manually rotate them as they expire. If you keep reducing the lifespan, to say 90 days, that starts to tip the scale. You'll be spending too much human time manually rotating certificates that it will make financial sense to just automate the process.

If the process is automated then revocation can be automatically handled as well (so long as ARI gains traction).