This is a misleading claim about the role of Russian troll activity on Twitter around the Brexit referendum. It is consistent with longstanding pro-Kremlin disinformation efforts to deny that Russia has engaged in electoral meddling, despite extensive proof to the contrary.
Twitter was initially criticised for its failure to provide an adequate response to the parliamentary committee seeking answers about Russian disinformation operations during Brexit. This did not mean that no evidence had been found of Russian efforts to influence the vote. In early 2018, Twitter admitted that Russian trolls had indeed targeted the Brexit vote during an evidence session with British MPs, who flew to the United States to question representatives from major technology firms as part of their fake news inquiry.
Further information came to light later in 2018, when Twitter released data showing that an army of Russian trolls sent thousands of messages with the hashtag #ReasonsToLeaveEU on the day of the Brexit vote. This army, which was linked to the Internet Research Agency (IRA), constituted approximately 3800 accounts.
For more disinformation cases about Brexit, see here.