I respectfully disagree. While there is communication overhead, it really does scale with the number of people you have to communicate with. If you have twice the people working half the time, you still have twice the communication cost.
Moreover, the communication overhead will eat up a much larger percentage of working time, since the person is only working half time. But you can't just communicate half as much. Any fixed overhead, such as status meetings, breaks, etc will suffer from the same effect.
Then there's the concept of flow. It's hard to get into a high productive state doing something like programming if you are working 4 hours a day, and 1 hour a day is overhead. Even if you get into that zone, you will benefit from it for a short period of time. For me, it can easily take half an hour to get my mind back to where I was on the previous day.
Ramping up and learning a codebase is also an overhead, and will take longer in terms of days (since you aren't working as long), take communication time, and then once you are an expert, you'll bring a smaller amount of time to bear against the problem.
In terms of teams, having a larger team (2x as large, let's say) means that you will get more good hires and more bad hires. But bad hires are far more of a problem than good hires, especially if they spend all the time communicating, asking questions, and making work for others. One bad hire can ruin a team. One good hire really cannot make a bad team into a good team.
All of this assumes also that all the part time people are working at the same time and able to communicate, which almost certainly won't be the case. Part time workers will not always be able to communicate, and may have to wait a while for an answer, further reducing productivity.
It's not only not negligible, but I worry it's not scalable.