OpenTimestamps doesn't do what DocuSign does. The only thing OpenTimestamps can prove is that a document existed in the past. That's it.

DocuSign provides a lot more evidence, starting with the fact that the document was signed! Right there the fact that it's hard to sign conflicting version of a document on DocuSign is a big improvement.

Of course, ideally DocuSign should also be time-stamping... But I suspect they will never do so because it would call into question their trustworthiness. Most of the evidence DocuSign collects is based on the courts trusting them to be truthful about things like what IP address/location was used to sign the document.