I am a bit skeptical of that claim, and am hoping that this is not a blind supposition based upon slave-ownership or lack of service in the regular Union Army. Any illumination on the issue in the form of primary source data that supports the claim would be welcomed.
The reasons for my skepticism--
1) The mansion was built in 1862 at a time when David Guitar was a captain in the local Union enrolled militia (61st EMM). My very extensive research of Civil War Columbia and Boone County has never came up with anything out of the ordinary in regard to the leadership of the local EMM (i.e., that some may have had Paw Paw tendencies)--quite the contrary given that the 61st EMM's commanding officer, Col. Joseph B. Douglass (later general), was the most prominent boots-on-the-ground EMM field officer and general officer in Missouri during the war. It's tough to get to that point by having enemy sympathizers serving with you.
2) The source material also tells us that Boone County southern sympathizers were being targeted for summary execution in 1864. Also, that Columbia itself was pretty solidly in the Union fold--it would be more than a bit breathtaking that a "southern sympathizer" would have invested a pile of money building a mansion there in the midst of the war when that mansion would have been a very likely target for a five gallon can of coal oil and a box of matches. A southern sympathizer's bankroll would have been more wisely invested by depositing it in a hole out in the woods somewhere than in building a show-case home in an area thick with Union troops.
3) Months after the end of hostilities David Guitar was serving side-by-side at conventions and on committees with his former c.o., Joseph B. Douglass, as well as other staunch Boone County Unionists such as William F. Switzler and James S. Rollins. This at a time when southern sympathizers were BARRED by law from such service, and only the "loyal" were permitted such service. These sanctions were not lifted until years later.
I welcome input on the matter. Perhaps even Mr. Sapp can weigh and clear up these doubts.