It's probably unwise to think prosecutors and federal agents are stupid. They were in the 1990s crypto wars, but not now. What we tend to perceive as 'stupid' is in reality, 'powerful.' They don't need to explain themselves, because they put the onus of compliance on you.
Gaming out the subpoena, Signal does not have this user information because it does not exist, but it does have server locations, 3rd party service providers relationships, and staff who can all be dragged into the process and system, where they can be charged with other arbitrary process crimes to put pressure on them.
It's a mistake to interpret any official action as a serial, single point transactional request. Like mice, if you think you see one, you have, and it's guaranteed there are many more behind it. Given where they have used the action to draw your attention, where in relief is the second part of the pinch or funnel they are creating?
If the legal system wants to destroy you, they can and do. Signal has antagonized them, and the current political climate is all about getting rid of any resistance to official powers and their unofficial private arms. Politically, there is ample incentive to take out Signal and cause users to switch to more amenable apps from friendly platform companies. They may even be able to compel friendly app stores to patch apps before they are distributed.
To me, this subpoena looks like the Cellebrite takedown was analogous to injuring a cop, where the response will likely be disproportionate and even extra-legal, because it is about maintaining public perception and belief.