Grant finally said so, even in his self-serving memoirs. Lee said so. Lincoln said so. And the historical record says so.
He prevailed in every engagement with which he was involved. He masterminded the "perfect battle:" The Battle of Nashville, the resounding Union victory that led directly to Appomattox. It is studied to this day, unlike the life-wasting blunderings of the drunken Grant.
He was a master of Training, Logistics, Signals, and Mobility. Under his command, Wilson's cavalry achieved the peak of civil war efficiency and fire power and Thomas integrated it closely with Infantry and Artillery better than any other commander. He cared little about was PR and intrigue, at which Sherman, Schofield, and their buddy Grant were masters.
Thomas was a great leader and humble man. Having said that, he was also a traitor to the South and Virginia. Post war he was not well liked by anyone, rightly or wrongly, nobody really likes a turn coat, even a useful one.
Thomas didn’t get a fair shake by the post war era or history.
But it pales in comparison to the lying war to protect the Marble Man waged against Longstreet. Early especially covered his failing by scapegoating Longstreet.