The answer to that can be found in their actual report (the key part of which I've highlighted):

"Although the Committee has no direct evidence, it considers that it is important to be aware of the potential for foreign interference in elections or referendums. "

In other words, they have no reason to think so, but want to sound tough anyway.

The most suspicious thing MPs have been able to point to is the collapse of the Government's official website people used to register to vote in the referendum just hours before the official deadline. The crash, MPs noted, was described as "technical in nature", but there were "indications" that it had been brought down in a mass cyber attack. Some might have thought - as the Cabinet Office says - that the website just couldn't handle the massive numbers of voters rushing to register at the last minute, as more than 50,000 were trying to access it at the same time in the final few hours.