One does not have to be a party to a federal crime, to be seen as attempting to influence witness testimony relevant to that crime.
Not a question in any legal sense.
RICO laws cover it, thoroughly.
Fair enough to be curious about it if you've never considered the issues before. But that is assuredly not the case for the federal investigators and prosecutors involved, and the laws on this are clear. That clarity is what you're pondering, but in fact is not in question, at all.
See:
United States v. Guadalupe, 402 F.3d 409, 412 (3d Cir. 2005)
United States v. Lopez, 372 F.3d 86, 91-92 (2d Cir. 2004)
A goon can be sent to intimidate a federal witness he's never seen before or has any direct connection to. That goon is still guilty of witness tampering. It's straighforward, boilerplate federal law.
The law is boilerplate and inclusive of the alleged actions. Proving motivation i.e. intent is the only ponderable, and whether the feds will get enough dots connected that they believe they can indict.
My reading on this is that there are not many dots that need to be connected, and Armstrong's motivations -- due to citation by the witness in GJ testimony as being involved in the commission of federal offenses -- is taken by the courts as largely self-evident.