Sometimes, it's a blue pill/red pill situation - prefer British English? United are training,
American? Jazz is in the play-offs (I honestly don't know if they are).
Myself, I tried and couldn't say "I don't know if it is" - a team is a group.
Other times, however, you might mean team as an entity - an organisation, then you'd say "is" - 'Luton Town is going into administration'
(I don't think it is, at least yet, it has been relegated into semi-professional ranks, though).

to everything already said, I'd like to add that yes, they're litotes, just slightly more awkward ones than say, "not unlike" - a phrase we're quite good and used to.