I am seeing this pattern elsewhere also. General anti-oxidants actually impair anti-cancer agents which work by correcting "electron flow"(i don't know a better word).Yes, very possible. This would also suggest that manganese supplementation may not be very good for people with cancer as it would increase anti-oxidant status. There is at least one study that found just that.
Cell Death and Disease - Manganese superoxide dismutase promotes anoikis resistance and tumor metastasis
"...Taken together with our previous finding that detached cells evade excessive ROS production by attenuating oxidative metabolism of glucose, we conclude that mammary epithelial cells coordinate their responses to detachment through increasing MnSOD and decreasing ROS generation from mitochondrial glucose oxidation, thereby mitigating anoikis. Anoikis is a barrier to tumor metastasis. Indeed, MnSOD expression is elevated in human breast cancer metastases compared with primary tumors. Expression of MnSOD correlates with histologic tumor grades in human cancer and contributes to cancer cell’s resistance to anoikis. Our study suggests that inhibition of ROS detoxification coupled with stimulation of glucose oxidative metabolism may be an efficient strategy to enhance anoikis and block metastasis."
In this study, Vitamin K2 Induces Mitochondria-Related Apoptosis in Human Bladder Cancer Cells via ROS and JNK/p38 MAPK Signal Pathways, Vitamin K2 induced apoptosis in bladder cancers but NAC actually saved the cancer cells meaning that selective ROS generation is required for killing cancer cells. However, the anti-oxidant defense recruitment in the cancer cells seem rather like a consequence than the cause. I think you once told about Carbonic anhydrase enzyme being raised in cancer ( i don't know where i read this info but the researchers proposed a diagram where hypoxia and other factors selectively raised CA IV in the cancer cells but nowhere else). I think this might be why you don't advise zinc supplementation as CA is a zinc driven enyzme , am i correct here ? :)