To get the 1% figure you have to include an unstated assumption, which is that a novel virus is just as likely to emerge from this lab as from all top 100 cities put together.

If we try to make a rough calculation from an unknowledgeable starting position, it might be more reasonable to say "just as likely to come from each 1M+ city as to come from each lab [of which there is one]". Then, if we didn't know the geographic origin, the lab probability is ~1% ( = 1/(100 cities + 1 lab)), but knowing the geographic start point you'd get 50%

P(this lab given Wuhan) = P(Wuhan given this lab) * P(lab) / P(Wuhan)

P(this lab given Wuhan) = 1 [this lab is in Wuhan!] * (1/101) / (2/101)