I think they may have misunderstood the meaning of the word tenure (note "with tenure") and not realised it meant only duration of employment here (versus its use in US academia).

There's also some weight in the word senior as that can also be referring to someone who's moved up within paybands within the same jobtitle as well as the (now less certain) expectation that someone who continues to work at the same place will get promotions to more senior positions as they show they're capable of them.