Botts was about thoroughly pro-Union as you could get. It is no surprise he would be pro-Union in his comments.
He dined on one occasion with Union Generals in Virginia during the war and was arrested shortly thereafter by Confederate forces for carrying a gun on an active battlefield. IIRC, Botts claimed he was carrying the gun for someone else and was released later the same day.
Going to the page you cited I came across some very uncomplimentary comments about Judah Benjamin—”a sneaking thief and perjurer, and an unconscionable traitor...”. I’ll have to spend some more time with the book. It looks entertaining, at the very least.