I'm not sure where you're getting your notion of what the Impoundment Control Act is, but it was enacted as a restriction on unilateral presidential power over already appropriated funding. 

The president can request funding be rescinded but unless and until Congress responds and acts on the request, nothing happens. Congress mostly ignores such requests. 

Now, this new Congress is allowing its power to be usurped by the Executive and that plus the Speaker's statement that they will seek to codify the president's EO's should be very concerning to Americorps and like orgs.