The 1st does have limits on it, it's not absolute. A soldier, for instance, who meets with an enemy soldier on the battlefield and gives up information is not protected by it. Those with a security clearance are not protected by it if they divulge information they are not supposed to divulge. Now, a member of Congress who goes to a foreign country and undermines the president's Constitutional authority to conduct foreign policy? I don't know that it's been adjudicated by the SC. I think it would be an interesting case for them to take.
You become a traitor when you go to a foreign country with which we are at war and undermine our country's foreign policy. We are not at war with Ukraine, so technically at least, the congresscritter is not a traitor, but certainly is not acting in the best interests of our country, and ultimately his own career. What happens if we end up at war with Ukraine in the near future? He's not going to be trusted with anything sensitive as he's already demonstrated he has no problem subverting official foreign policy.