The algorithm was neither proposed nor designed by the US government, it was made by (what is now known as) Signal, a 501c nonprofit.
The claims of signal being "state-sponsored" come from assuming how money flows through the OTF - Open Tech Fund - which has gotten grants from government programs before. (IIRC)
It wouldn't make sense for the US Gov. to make such a grant to make a flawed protocol, as any backdoor they introduce for themselves would work for any outside attacker too - it's mathematics. It works for everyone or for no one. Would they really wanna make tools that they themselves use, just to have it backdoored by other state actors?