Wasn't the Smallville Green Arrow among the fallen (dead?) heroes seen lying about the battlefield back in the Elseworlds teaser seen from last year?
On the 90s Flash Earth?
Yes, the Smallville
Green Arrow costume, among other DC props, appeared on Earth-90 in "Elseworlds". But given how sci-fi TV properties usually work, we can probably discount that as an Easter egg for fun courtesy of shared assets. Like the Smallville
Kent farm being reused for an alternate reality version on Supergirl
I predict that "Crisis on Infinite Earths" will logically clarify 1990 The Flash
and 2004 Smallville
as based in separate realities.