Exactly this, the "spec" is almost always a very rough draft that is largely written and consumed by people that want to feel like they contribute to the project even though they don't understand the customer or the developer situation that well. You might reference it a bit in your first offering and then ignore it as the stakeholder sees what the spec produces and realizes the spec wasn't really what they wanted when they see it live.
Once upon a time more weight was given to design, but the industry largely realized that all that very careful effort just became a liability of sunk cost fallacy when they realized the resultant output was not desired, but so much work had gone into the spec we don't want to change.
Nowadays it's a way that PMP minded folks feel like they are core technical contributors without learning to code. This is of course the target audience. Spoke to an executive that sincerely believes the only thing of irreplaceable value is his 'insight' and over 90% of his employees are going to be dismissed since he can just do it all himself. In practice his is the *first* job that could go to LLM, as all he ever says is either obvious stuff or just confidently wrong and his business decisions amount to "all we need is more customers and for them to pay more for it and we will be profitable"... Genius.
