Legitimacy can mean more than just following the letter of the rules. There's a pretty good argument to be made that refusing to even hold hearings for a nomine

Legitimacy can mean more than just following the letter of the rules. There's a pretty good argument to be made that refusing to even hold hearings for a nominee is a violation of the Senate's Constitutional duties. And refusing to uphold norms is a completely reasonable basis for calling something illegitimate as well. A pretty big chunk of our legal system is based on precedent and norms rather than written law.