The link is to a playlist of 3 videos. All 3 show that Lux's ult can refund its cooldown multiple times.
The first is from the 3/23 PBE. Lux has Presence of Mind, so it's harder to tell, but the cooldown on her ult is reduced by more than it should be.
The second is from the 3/26 PBE, no Presence of Mind. At level 10, the built in cooldown refund on the ult should be 30%. The ult's full cooldown is 48s, and after the kills register, it drops to 19s. That's about 60% reduction, or what you would expect if the ult cooldown refund triggered twice.
The third is from the 3/27 PBE, no Presence of Mind. The two kills are separated in time, so you can clearly see the two refunds to the ult cooldown.
I don't know if 3+ kills refunds more than 2.