I'm a fan of strength in stones too because there's no fun in Elo, though I don't think my wish (oldstyle Jubango match between human and AI) will come true

Anyway, we have a new information on this from Reddit AMA
David Silver: We haven't played handicap games against human players - we really wanted to focus on even games which after all are the real game of Go. However, it was useful to test different versions of AlphaGo against each other under handicap conditions. Using names of major versions from Zero paper, AlphaGo Master > AlphaGo Lee > AlphaGo Fan, each version defeated its predecessor with 3 handicap stones. But there are some caveats to this evaluation, as the networks were not specifically trained for handicap play. Also since AlphaGo is trained by self-play, it is specially good at defeating weaker versions of itself. So I don't think we can generalise these results to human handicap games in any meaningful way.
Now back to my opinion. We all know that the 'real world gap' between AlphaFan and AlphaLee are two full stones rather than three. So let me make an unscientific big assumption that the 'real world gap' between AlphaLee and Master are also two full stones too. So the new arrangement (relatively to KGS ranking system) is
AlphaFan 9d
AlphaLee 11d
Master 13d
Now while Zero can probably give Master one full handicap stone (it's 327 elo stronger than Master, already calibrate down from its 89% winrate against Master which account for 363 elo difference) , the 'real world gap' assumes it can only give a KGS 13d human player no komi advantage. So the rank might be
Zero 13.7d
That means Zero can give KGS 9d human opponents no komi with four additional free moves, an H5 game in KGS term. Ke Jie is estimated to be KGS 11d so Zero can only play an H3 game with him.
I think this is good enough to claim that Zero is the strongest go player in history even in relative strength (that determined by how big the gap is between no.1 player and players below him). It's not gonna be as impactful as Go Seigen for sure. But in Reddit AMA David Silver also hint that the 'AlphaGo tool' is work in progress.
PS. AMA also reveal in Chinese rule White has 55% winrate according to AlphaGo. In Japanese rule Hideki Kato said DeepZen also slightly favor white (not black as I assumed at first). So I think AI treat komi as precious free point.