As I said, I don't know if that's really what happened or how it was calculated, but it is certainly plausible for a such a statistic to occur. Indeed, I think that was the GOAL!
One of the early suggestions on these threads was that FP+ would be used to manipulate guest behavior into riding attractions that they otherwise might skip. (Something Disney has tried in the past with “bonus” FastPasses.)
The difficulty in reconciling this with FP+ is that most popular attractions cycle with all capacity used. Attractions such as Hall of the Presidents or Carrousel of Progress might have unused capacity during peak holiday season (as Iger specifically mentioned) but they don’t have FP+. Therefore, it’s challenging to envision how this could be used to claim that Disney was “able to accommodate about 3,000 more additional guests in the Magic Kingdom per day thanks to MyMagic+.”
MM+ does not increase the physical size of MK. It does not add ride capacity. It does not increase restaurant space. It might let guests enter the park a few seconds faster but when MK is in a phase closing as happens during peak holiday season, faster entry into the parks is not a factor.
One possible interpretation of the “3,000 more additional guests” claim can be reached by taking into account what Iger discussed next, namely FP+.
“What we are seeing there is substantially higher utilization of that product among our guests than we saw with the traditional FASTPASS. By the way by a wide margin.”
When comparing FP+ vs. FP, FP+ is able to accommodate about 3,000 more guests than FP. However, this is only because FP+ robbed capacity away from Standby lines.
The net gain of FP+ is 0, a concept that Iger did not disclose during the call, presumably either because he wanted to convince analysts that MM+ is beneficial or because he simply failed to grasp the concept of where FP+’s “substantially higher utilization” came from.
Something else I've considered is increased staff. Simply put, WDW reportedly has hired additional CMs to support MM+. Might the claim of 3,000 more guests accommodated simply be a reflection of more employees on hand (as a result of MM+) that allow for more guests to enter the park? In other words, is MK's capacity at least partially determined by a minimum CM-to-guest ratio?