You bet they can and should refuse to offer any testimony. NTSB is playing hardball, and they are facing a min of a job action, and a maximum(depending on what they say) of potential criminal charges down the line. The US govt is no longer the kind older uncle. They are going for blood by issuing a subpoena for testimony. The only thing I would say under oath is 'I refuse to answer, and assert my constitutional rights', even if they aren't citizens.
This kind of stuff is BS, and needs to stop. It most certainly will not benefit safety. The pilots crossing without authorization are going to take a rip. Get it done, and move on. Not just the 5th Amendment at work here, but we also have something called the 'Pilot bill of rights' which offers significant legal protection. I am almost certain both pilots of the offending runway plane have counsel already. If I were their atty(solicitor) I would seek to quash(remove for cause) the subpoena due to missing any criminal intent. The pilots offered to give information, but not enough for the feds. Tell them to pound sand, it's not going to get better with age.