I think that overselling is a trait of young companies, really.
Every sale matters when you don't have customers, and if it means a little back-breaking work to get that first bit of revenue, I know very few companies with the fortitude to say 'No'.
Once the product has established a foothold, reputation, or repeatable sales process that works, product managers get an idea of exactly what it is that customers want, and across more than one data point. From that, they're able to communicate the exact value they provide to the sales team, and that sales team is able to approach customers with an actual, legitimate, proven value proposition.
It isn't until the product team really knows what they have that those requests become easier to turn down.
(Or at least, that's been my experience.)