I spent some time on it and here is what I understand.

Indeed Math. Random is called when a private key is generated, however, it's called many times in a loop. The result of math.random() varies because of the state variable used in Math.Random varies every time it's called. (The implementation of math.random depends on the browser)

 while(rng_pptr < rng_psize) {  // extract some randomness from Math.random()
    t = Math.floor(65536 * Math.random());
    rng_pool[rng_pptr++] = t >>> 8;
    rng_pool[rng_pptr++] = t & 255;


This in turn is mixed with the time millisecond when the key is generated, I think there could be a small space to search when we know the exact time of key generation otherwise, I am assuming that the keyspace is large to search.