With a clicky switch I expect it to actuate when I hear it. With a tactile switch I expect it to actuate when I feel it. With a linear switch it depends on the switch. You can get linear switches that actuate as soon as you'd like.

The more I think about this the more useless it seems. Several of the keyboards he tested come with your choice of switch, different switches with different actuation points, and so you can't just say Keyboard X has a latency of Y without mentioning what switch you were using if you're going to measure from the beginning of the keypress.