There is no science behind it. Testing saliva pH is just as worthless. The actions of mouth bacteria and diet will influence saliva pH more than any metabolic effects.
The assumptions about pH and cancer status were discounted over 40 years ago. Low acidity is supposed to predispose to low oxygen (anerobic) which is supposedly what cancer cells require.
Any researcher can tell you that cancer cells love oxygen, perhaps more than non-cancer cells. And again, most studies show that high alkalinity predisposes to cancer, not protect against it.(1)
I generally don't get involved in discussions about other modalities, but I refuse to allow any assertion that this is 'scientific' go unopposed.
1.Food Chem Toxicol 1987 May;25(5):359-62 Effects of pH on the carcinogenicity of o-phenylphenol and sodium o-phenylphenate in the rat urinary bladder.Fujii T, Nakamura K, Hiraga K.