Tbh I think this definition doesn’t really reflect the meaning of the word. Contender would be someone who can genuinely compete for the title aka a team who has a genuine chance at winning it, which I think the Bulls did.
The previous season would be irrelevant. A team could have the 10 best players in one season and then the worst 10 in the next, so it’d be silly to judge them as a combination, one squad would likely be a contender and the other wouldn’t.
It’s hard to compare with the PL because the NBA has playoffs after the regular season. But yes, I’d say as the 2015-16 season went on, Leicester were eventually contenders, although they wouldn’t have been predicted as such before the season but that’s not the only way to determine a contender imo.
And like Aussie said, the Bulls actually did string together at least two good seasons anyway.