Of all the things that aren't evidence, an indictment is possibly among the most not-evidence things.
A judge famously said you could indict a ham sandwich. In practice, grand juries say whatever a prosecutor wants them to say.
And as Russian nationals, it's also unlikely they'll ever have to actually stand trial, which is when evidence would be required to enter the public record to convict them.
I also very much doubt the US government will want to go through a discovery phase with these people, so I'm not sure the DoJ really wants a trial to actually take place.
Either you "protect sources and methods", and not make strong public accusations, or you make accusations and disclose evidence.
Doing one but not the other is indistinguishable from just making false claims - which the US is known to have done repeatedly in the past (another example - Gulf of Tonkin).
A judge famously said you could indict a ham sandwich. In practice, grand juries say whatever a prosecutor wants them to say.
And as Russian nationals, it's also unlikely they'll ever have to actually stand trial, which is when evidence would be required to enter the public record to convict them.