Earth-90 counterpart of John Diggle ( from Earth One ) is Green Lantern John Stewart ?!

No, just the counterpart of some guy who wears a ring.

Yes, that was clearly what we were meant to infer from that.

Will it go anywhere? Probably not, I think it was just a cool Easter egg. But still, it was a pretty fun moment.

Ring can mean anything. Sure there are many easter eggs in entire shows about Green Lantern, John Diggle is black guy and saying "ring" can probably mean this but let wait

What else could it mean? I seriously doubt he was talking about a wedding ring or something, whatever ring he was talking about had to be important, and I can't think of what else it would be.

That's definitely what they were alluding to. It's been a fan theory since the first year of the Arrowverse, even back when the Arrowverse was entirely gritty/realistic and there seemed like no prospects of a GL coming into play. It's probably just gonna stay an offhand reference though, a Green Lantern would be almost impossible to pull off on the CW's budget.

Iirc in one of the previous seasons dig was sitting at a desk and his name said “John S. Diggle” obviously alluding to him having “Stewart” as his middle name. With the recent comment on his ring I’d say yes. They both are African American, both high ranked military officers, both work closely with the green arrow. Both are tough, awesome, no nonsense characters. I think it’s obvious they are implying he is a version of John Stewart

@mrmonster: one of my buddies instantly thought it was referring to to diggle being a flash on earth 90 but that seems absurd to me, the whole crossover has been fan service, and a big fan theory has been that dig and John Stewart may be the same, ppl have been photoshopping him as a lantern since the show started airing, I think it’s obvious they meant for it to be a lantern reference

I loved this little Easter Egg, it was delivered well.

I never even consider Diggle as a Green Lantern until the guys at Blind Wave suggested it, so I thought it was really cool they added that into the lore. This whole crossover was kind of a "Having your cake and eating it too", especially with how they handled the Batman situation between Arrow's and Supergirl's Earths.

I don't know whether this was sarcasm or not

Probably. Though I wouldn't want to see it.