They get this fictitious specific impulse by scaling the effective exhaust velocity by a scaling parameter to account for the fact that the exhaust mass flow consists of extra mass (air), in addition to what is carried onboard (fuel). This specific impulse is still used for comparing jet engines based on efficiency. Another use for it is to calculate space mission requirements in launchers utilizing air-breathing engines in their first stage (as your parent commenter points out). Though such vehicles don't exist yet, there are concepts being pursued. Some of them use a scramjet and others are more elaborate like the (cancelled?) SABRE engine. So those Isps are not completely meaningless.

The general idea is that you can get much better results in terms of deltav if you can find at least part of the reaction mass from elsewhere without carrying it onboard. Even inert nitrogen is useful as a reaction mass. Another way to get a good result is to use separate sources of reaction mass and energy. Then use that energy to accelerate the reaction mass as much as possible, so that you get a decent deltav by the time you exhaust the reaction mass. This is what ion and plasma thrusters do.