It is simple. 500 MHz option is being sold as after sales, user installable software option. You enter license and it unlocks scope (any SDS2000x+) to a fully featured , CALIBRATED 500 MHz version. That means every single one was calibrated and tested to 500 MHz version standards, and then by software configuration it was delegated what bandwidth it will have.
Bit of nitpicking, but actually the datasheet only states that the 350MHz model SDS2354X can be updated to 500MHz. It's a least possible that Siglent won't
sell you an upgrade to 500MHz if your base model is the 100MHz or 200MHz model since they know that these models were not tested/calibrated for 500MHz. The fact that the scopes accept the update doesn't necessarily mean that this is really something officially supported by Siglent. As an example: the SDS5034X accepts the upgrade key for 1GHz but Siglent won't sell you this key knowing that the frontend won't reach 1GHz.
And since I already touched this topic:
It makes me think of the first generation sds5034x frontend which don't have the same hardware as the 500 and 1GHz.
IMHO it's still just an assumption that this ever changed. Or is there any proof in the meantime? At least the sections in the datasheet differentiating the 350MHz model remained unchanged. Or does Siglent sell an upgrade to 1HGz for the 350MHz model in the meantime (e.g. based on serial number)? If they don't: why shouldn't they unless there is a technical limitation?
Besides, it's not uncommon to use different placement for the 1GHz models. AFAIK Tek and Keysight did that in the past. And while different placement means different HW, it's not nearly as expensive as having a different PCB (layout).