[boycott]

That’s not complete. Note McCain, Mitch, etc. .....

https://www.govtrack.us/congress/committees/SLIN

[Robert DeLong]

ex officio - Literally, by virtue of one's office. The term refers to the practice under Senate rules that allows the chairman and ranking minority member of a committee to participate in any of the subcommittees of that committee, but generally not to vote.

SOURCE

So the question is, were they allowed to vote on this issue? I cannot find the vote count and who voted which way to determine if they voted. Chances are pretty good that McCain did not vote.

Also though, government track is not a government website, whereas my source was. So perhaps that is why they do not include ex officios.