I'm unsure how to interpret that は in that part of the sentence. I'm assuming it's a contrastive は but I'm not sure what it's contrasting.
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That wa is not really contrastive but anyway, in general, the particle tends to be wa when the sentence is negation and
especially when the word doesn't feel really unexpected against the context to the speaker. To be honest, either ga or wa are fine as long as the example sentence is concerned.
(As for the controversy about wa's essential role, If you support contrastive wa fundamentalism, it is. If you support topic marker wa fundamentalism, it is.)