Of course, this debate has been going on, and will continue for a long, long, time. As usual, SM appears to present a logical argument, but on closer inspection, it doesn't hold up. And here is why:
An existential claim (that of something existing) is a binary proposal. It either does, or it doesn't. Like being alive or dead, pregnant or not pregnant. A theist claims that a deity exists. They make no scientific claim about this (if they are smart), but rely on faith in the unprovable. A skeptic may say "I don't believe you unless you have proof". That's actually a good position (scientifically), and is consistent with the agnostic position. In this case, the theist makes what is known in philosophy as a "truth claim". The agnostic denies the truth claim is supported, and therefore withholds assent.
The atheist position, on the other hand is not the same as the agnostic's in this example. In this case, the atheist makes the truth claim that a deity does not exist (even if they don't admit it). Otherwise, they would be an agnostic. Remember that an existential claim is a binary proposal.
But it is illogical to reject a deity, then hedge that it might exist.