The sample needs to be a certain size AND must also be representative of the group being sampled to yield accurate projections. Larger sample sizes generally produce better results, but the only accurate result is counting every user. Think how politcal polls are manipulated (oversample democrats then declare majority of likely voters support their democrat cadidate). They carefully contrive the sample to ensure the desired result to use as a tool to manipulate public opinion. For sure twitter carefully selected the 100 accounts to yield a number they felt the public would believe, in this case 5% bots.

The sample needs to be a certain size AND must also be representative of the group being sampled to yield accurate projections. Larger sample sizes generally produce better results, but the only accurate result is counting every user. Think how politcal polls are manipulated. They carefully contrive the sample to ensure the desired result to use as a tool to manipulate public opinion. For sure twitter carefully selected the 100 accounts to yield a number they felt the public would believe, in this case 5% bots.