I got the same thing and I got an interview the day after. Looking through last years and this years thread, it seems to hold true that that status means impending interview!
What it looks like to me is this is the usual for people on hold who get an invite (and this is just a thought):
Application complete status>Hold for re-review>under review = invite
Most rejections didn't get "under review" at all, and theres also people who didn't get a hold at all and went straight to interview.
My theory is that theres an initial committee that goes through all the apps, and when they find someone they like, they send it to a second committee that offers the invite. When you're "under review", you've been forwarded to the second committee, but if you're in hold, the first committee is still determining whether to forward you to the second committee. This might be why updates are so helpful--when the first committee sees a new update that tips the favor toward you being a good candidate, they may forward you to the second committee. The second committee confirms that you should receive an interview, hence the "under review" status.
This is all conjecture though...