It was eligible. The nominations were a joke. The Oliviers love to give awards to big name actors. They often snub the RSC completely. The judges are comprised of two groups, industry members and members of the public. The judges in the former category are not neutral parties and often producers etc. of eligible shows are the ones doing the judging and naturally vote for their own shows.
Some of the nominations this year went to very average and not particularly well-reviewed performances, while other remarkable and acclaimed performances like DT’s RII, Simon Russell Beale’s Lear, Ben Wishaw in Mojo, and Adrian Lester’s Othello were snubbed.
For what it’s worth here is a post from a former member of the Olivier judging panel discussing their experience of the judging process.
"The panel were unanimous that [Jonathan Slinger’s) Richard II was the most remarkable performance of the year; some, including me, found it one of the most remarkable we’d ever seen. We couldn’t believe that an organisation of supposed theatre professionals could have seen Slinger’s performance and not deemed it worthy of even being shortlisted. The award was eventually won by Jacobi for his Malvolio, an okay/adequate performance vastly inferior to Slinger’s. At one point during panel discussions one of the Oliviers staff said (re Jacobi) "Well, he’d be terribly upset to not be at least nominated". They also said re Slinger "They probably won’t know who he is."
They were also evasive regarding the SOLT members’ actual attendance of all productions.”