Judokas aren't as aware of the guard game as they should be, I too find it's far easier to armbar/triangle etc from guard in Judo class than in BJJ. The more Judo people learn from people who cross train in sub grappling or BJJ, like you (Liffguard), the better off they, and Judo, will be. So you should definitely share.
Alternatively just crank on the armbar repeatedly, but unless you explain why it was so damned easy, you won't be helping your training partners much (but I know you know that already).
Thanks all. That's a lot of helpful feedback.