There's nothing about lingering per se that makes it harder to predict. They're going to have a predictable average meal length as well, just longer than yours.
What might make it harder is the standard deviation - in other words, how much variation there is between how long it takes. For example, if they know that 90% of the tables finish within 80 to 90 minutes (a ten minute range), they can predict more reliably that a different restaurant in different circumstances where all they know is that 90% of the tables finish between 60 and 80 minutes (a 20 minute range), even though the average time is less as the second restaurant.
But Disney, unlike the typical standalone restaurant, can afford to hire the statisticians and queuing theory experts to figure this out. And even when they are hit with a worse variation, they can still take action. Suppose their ADR statistician says "In Sep., we normally operate with 90% confidence that 98% of all ADR holders will have a wait of less than 10 minutes. But in late Dec., our standard process only has 85% confidence because we know from our carefully gathered metrics over the last ten years that the variance increases in Dec." What do you do? You reduce the number of ADRs. It's really that simple. Disney, unlike the typical restaurant, has a huge advantage for their park restaurants. They can safely reduce the ADRs without any loss in throughput or revenue, by relying on walk-ups. Or at least they could, before they allowed the ADR system to destroy the walk-up approach.
This of course assumes that there really is an increase in variation. It's certainly a plausible conjecture, but I don't think we know for sure that lingering increases variation, just the average (predictable) table time. Even this being the last year of Osborne will just increase the total number of people in the park, but not necessarily the variation.
TL;DR: Longer table times don't mean less predictability, but variation in table times does. Yet Disney can still predict that their estimates are less reliable, and compensate by reducing ADRs.