It is very, very unlikely that any such different-model swap would work, because the other electronics in the camera are tied intimately to the sensor. In the case of very similar models (one generation to the next, say), it's possible that all of that could be swapped in and still physically fit, but even that seems unlikely as improvements (and cost-saving measures!) often change the form factor as the design evolves.

There is certainly no market for knock-off or third-party fake sensor modules (as one might find with, say, power supply bricks). The market and margins just don't make that worth it.

So, I think it's likely you're getting what they say.

On the other hand, I'd urge you to be cautious about fetishizing one particular sensor make, model, or generation. All sensors in models of this level are of exceptional quality, and it's incredibly rare that there's an actual photographic situation where the minute differences between A+++ and A++++ actually have a visual (let alone artistic) impact on the results.