Ah, I was just making up numbers, but since those car jump start battery packs claim to have hundreds of amps (I saw one with 1,000 in a quick search) at 12V, and this will be like a bunch of those, if those numbers are real it should be totally feasible for it to dump the energy fast enough.
Since you brought it up, I might as well check the other numbers.
Doing the 30 seconds multiplication for the half gallon I mentioned (my kettle only holds 1.7 l = .37 gal anyway), that’s 208.3 Wh, which for a typical lithium 3.6V nominal voltage would be 58Ah, a lot of battery, but not unrealistic - I’m holding a power tool battery (Stihl AK30) which is 180 Wh (50 Ah)
Over the course of those 30 seconds it could also pull 12.5 Wh from the wall to slightly reduce the needed battery size.