The reason Tang xiu expected them only to be white level of martial artist was because unlike he who had Spiritual Conversion Flower which could help him cultivate spiritual energy they had profound energy. The Spiritual energy was countless time more beneficial then Profound energy for example a martial artist who was at Gold level of Martial artist needed at least 60 years to advance by using only profound energy but by using Spiritual energy a martial artist would not even need 10 years. It is simply comparing a dirt with Diamond so the benefits given by both the spiritual energy and profound energy were different but the problem was the profound energy was completely harmless to anybody so it was completely safe cultivating profound energy but when cultivating spiritual energy provided by the Spiritual stones they had to be careful not destroying their cultivation as a whole so one had to be Gold level marital artist to be able to cultivate the False Spiritual energy. The false spiritual energy was the spiritual energy provided by the spiritual stones but they were unable to give true benefits of Spiritual energy.
At the cave
"Do these brats think they can be undetected by me who was once a Divine king" Tang xiu thought as a former Divine king in his past life he had excellent detection ability
Tang xiu said "OK all of you come out now even though you have exceeded my expectations by teaching Mastery stage of the [Phantom shadow techniques] you still are faraway from surpassing me in terms of hiding "
After he said people started coming infront of him and said "Young lord how were you able to detect us???"
Tang xiu said in arrogant voice "Tch aren't you thinking lowly of me"
"No we dont dare"all of them said
Tang xiu asked "Tell me how are all you??"
"Young lord as you promised us out injuries are healing at unbelievable speed" the slaves which were injured said
By seeing all of their situation Tang xiu evaluated there were total of 215 person who were able to reach white level of marital art but other were not very far from his estimation all of them would reach white stage of marital art in less then one week.
"It seems it is right time to give you knowledge about Marital art"
After explaining them
"Milord why is this not known to the world"
Tang xiu replied "It is probably that the martial artist are hiding and building their own strength"
"Then what is the highest level a martial artist can achieve and the limit a marital artist can achieve??" asked Shadow two
"It is a good question" after some thought Tang xiu said "The limit of a martial artist can probably increase time to time because as in the past the Gold level marital artist used to known as highest peak but as the time passed the highest peak rised to the point there are beings known as Divine being,Divine king,Divine Emperor,and lastly Mythical being and when I analyze all the things I have concluded that the limit of a Marital artist will increase as long as Marital artist generation will pass down from one generation to another generation creating geniuses that will make a goal to surpass eachother and in the end will create new limit of Martial art"