Anyone who has any experience with centralized databases, would be able to tell you how useless sealed sender is. With message recipients and timestamps, it'd be trivial to discover who the senders are.

Also, signal has always had a cozy relationship with the US (radio free asia was it's initial funder) . After yasha levine posted an article critical of signal a few years back, RFA even tried to do damage control at a privacy conference on signal s behalf:

Libby Liu, president of Radio Free Asia stated:

Our primary interest is to make sure the extended OTF network and the Internet Freedom community are not spooked by the [Yasha Levine’s] article (no pun intended). Fortunately all the major players in the community are together in Valencia this week - and report out from there indicates they remain comfortable with OTF/RFA.

These are high-up US government employees trying to further spread signal.

You can read more about this here.