Your scenario is confusingly formulated, but it ultimately deals with moral risk.

If you suspect that it's fraud, you should seriously consider whether you want to be a part of it, even if you can't prove the fraud. Examples are companies centered around cryptocurrencies: not all of them end up being rugpulls, but enough of them do, making every new company potentially morally risky for each engineer.

If you only suspect that it's an overly ambitious project, but not fraud, the moral risk is obviously lower. Sure, it may be economically risky - as all unproven business models are - or you may discover the fraud later (like the author apparently did). Examples are startups like SpaceX in its initial stages: there was nothing ostensibly fraudulent in trying to make and sell better rockets, just very economically risky.

Obviously, the devil is in the details, but if you're honest with yourself, you should do your due diligence (i.e., does it seem like fraud or not) and determine your moral risk appetite. It's up to the wider society to reduce and punish the unacceptable levels of moral risk, regardless of the appetites of individual engineers.