On the contrary, he is probably quite aware. Immigration's advantages already come with tiny numbers of immigrants, whereas most problems caused by immigration

On the contrary, he is probably quite aware. Immigration's advantages already come with tiny numbers of immigrants, whereas most problems caused by immigration get caused by large sudden influxes of immigrants. So he's taking a tiny bit of immigration to illustrate that an influx at over 6 times won't generate any problems (I think you underestimate the difference. It's actually about 10x)