sounds reasonable
It’s not just a supposition, there is quite a bit of evidence to support that view. Some people on reddit of all places dug into the documents when they first came out and that was the general conclusion most people investigating it came to.
One example of something that points to that:
Assange announced that he would be releasing the documents first, before anyone had ever heard of “Guccifer 2.0”. Then, this “Guccifer 2.0” character shows up and claims to the be the source, and releases some of the DNC documents, ahead of Assange, but he ONLY released documents on topics that Assange had mentioned in his press release. When Assange later releases the full DNC emails, there are a ton of of other documents that were not mentioned in the original press release, and were not included in what Guccifer 2.0 posted. So it would appear that whoever Guccifer 2.0 was, he didn’t know exactly what emails were stolen, and was just releasing stuff that Assange had mentioned in order to bolster his credibility without accidentally releasing new documents that Assange didn’t have.
There’s some harder evidence when you dig down into the metadata of both sets of documents that were released. They are actually different sets of files saved by different people, not one set of data that was obtained by Guccifer 2.0 and then copied by Assange.