The RNC Chairman's assertion is important because the Democrats are already trying to make it seem like these emails are not legitimate. They are making statements that imply someone else is behind this instead of taking responsibility for pushing the agenda of Clinton and doing everything possible to make sure Sanders didn't win.
The DNC is trying to shift blame in any way that they can for the leaked emails. The RNC chairman's assertion that the Russians did not write the emails is relevant because it puts the responsibility back on the DNC. They are trying to make light of a serious situation so that people will focus on the election and not the email leaks.
While the RCN chairman has said many rediculous things, this was not one of them. The Russians had very little to do with the e-mails, and it was a scapegoat move and wrong for the DNC to blame the e-mail scandal on the Russians. They should have accepted responsibility for them.
The relevant issue at hand, with the DNC email leak, is that this was an underhanded attack from the RNC and associates. The DNC is still at fault in many ways, but the RNC are the ones who have pushed this forward as a prominent issue regarding the nearing Presidential race.