Flynn cooperated with Mueller regarding contacts with Russians during the “transition”

quote:

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Mueller says Michael Flynn gave 'first-hand' details of Trump transition team contacts with Russians

-Former national security advisor Michael Flynn has given special counsel Robert Mueller "first-hand" details of contacts between President Donald Trump's transition team and Russian government officials, Mueller said in a bombshell court document filed Tuesday night.
-Mueller, in that sentencing memo, said Flynn's "substantial assistance" to the special counsel's probe over the past year warrants a criminal sentence for him on the low end of federal guidlines, which could include no jail time.
-The memo relates to lies Flynn told FBI agents about his conversations with Russia's then-ambassador to the United States, Sergey Kislyak, during the presidential transition.

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LINK

So Flynn told Mueller’s people about contacts with the Russians which happened AFTER the election.

How is that collusion to influence the election?

Eta: Shouldn’t an incoming administration start communicating with other governments during the transition period? What’s the “bombshell” here???