There is a big difference here. Grusch has given specific locations and names of people that congress can subpoena. Marco Rubio has confirmed that other highly credentialed whistleblowers with first hand knowledge are confirming Grusch’s claims. I believe we will know soon enough whether this is real or not.
And Rubio has said (intentionally or not) very demonstrably false things in the past. I wouldn’t trust him to always be accurate.
Let’s just look at the facts here. Grusch has given congressmen locations of where these alleged craft are held, program names that the SAPs are hiding under, and names of people involved. Congress could very easily get to the bottom of this by appointing a special envoy with a full top secret SCI clearance to go investigate these locations, and they could subpoena the people that Grusch has pointed out who are involved with the programs. This could be resolved in a week if they would just do that, so why not let them investigate this really quickly?