Lightning is a connection from cloud to earth borne
charges. Electricity travels through kilometers of
non-conductive air. Why would some millimeters of air between
two RCD contacts stop same electricity? 1,000,000 mm of air
did not stop lightning. Why would 4 mm of RCD air stop what a
million mm could not?

The bottom line is this. Nothing is going to stop
lightning. Lightning protection is about shunting - also
called diverting, redirecting, or electrically connecting - to
earth ground. Any protector that claims to stop or block
lightning (such as in the RCD question) is simply promoting a
myth. And yet that is exactly what many people do - promote
the myth - when they recommend plug-in protectors.

In response to the OP's question. 4 mm of air inside the
RCD (open switch contacts) is not going to stop a potentially
destructive surge.

Richard Herring wrote:
In message , w_tom
Again an assumption that something will stop or block a
destructive transient. Kilometers of air could not stop the
transient.

I think you need to be more quantitative here. Kilometres of
air could not stop what, exactly? They certainly do a good
job of protecting me from lightning strikes in the next
village.