That empty space bit does not say relative to empty space. What he means, and it's taken to mean by every other person, and physicist (you know, people who learned this from him and his students?) was:

Empty space = VACUUM.

There is no magnetic or electrical resistance, so you use the permisivity and permeability of free space constants. This is as opposed to the values for those constant when measure in say, ohh, a dielectric (aka electrically insulating material). The change in these constants is the entire reason why light undergoes refraction through various materials.