Yes. Fouled bad enough not to be firing all the time leaves raw gas in the pipe. That doesn't count as in that doesn't generate any O2, so the O2 sensor see lean, and then the ecm commands a rich correction.
Since the O2 only senses oxygen, if the fuel is not fired, then fuel and air would be pushed out of the cylinder (with the fouled plug), the O2 will sense O2 (from the air) and try to drive the mixture richer, to no avail. So, Bruce's assessment is correct.