It's not unreasonable to ask but it's completely unreasonable to expect that anyone pumping a coin on HN would have even the slightest inclination to be truthful about their position.
The disclaimer (truthful or not) serves two purposes:
1) For the pumper poster, it makes them more conscious of the lie. Promoting a coin and forgetting to mention your holdings feels like a little white lie of omission. Actively lying about your holdings feels like deceit.
2) For the reader, it’s an important reminder that a seemingly objective technical-looking comment can be motivated by greed.
