More accurately, it was a martial art in the 19th century, when it was practiced by professional bodyguards, but that did not resemble much what is today called Tai Chi.
What is practiced today is a health-preserving exercise derived from that martial art.
This is the same as for what is today called "karate", whose origin is from a 19th century martial art practiced by professionals, i.e. bodyguards and law-enforcement officials, but from that in the 20th century health-preserving exercises and sportive competitions were derived, which do not look at all like the original art.
There are people who claim that they still know which were the martial art techniques from which the movements of the Tai Chi forms were derived, but at least for most of them the claims are unlikely to be true and if they demonstrate a supposedly original movement there usually exists no evidence that it truly was transmitted from the ancient masters and it is not just a modern guess of how the original movement might have been.
As a health-preserving exercise, Tai Chi is indeed one of the best.
What is practiced today is a health-preserving exercise derived from that martial art.
This is the same as for what is today called "karate", whose origin is from a 19th century martial art practiced by professionals, i.e. bodyguards and law-enforcement officials, but from that in the 20th century health-preserving exercises and sportive competitions were derived, which do not look at all like the original art.
There are people who claim that they still know which were the martial art techniques from which the movements of the Tai Chi forms were derived, but at least for most of them the claims are unlikely to be true and if they demonstrate a supposedly original movement there usually exists no evidence that it truly was transmitted from the ancient masters and it is not just a modern guess of how the original movement might have been.
As a health-preserving exercise, Tai Chi is indeed one of the best.