I don't have any idea of why "lab->flanker(J-20)" or "lab->IL76->flanker(J-20)", but from a pure Chinese language/terminology standpoint, the original text (hightlighted in yellow below) is very likely excluding any usage of IL76.
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In English and your interpretation, both flanker and IL76 are testbed because they are not the target airframe, J-20 is the target airframe. However, in Chinese, 装机 literally means "mounting on the aircraft". Now IL76 is an aircraft too, but for IL76 there is a specific term in Chinese "实验平台" literally meaning "test platform" or testbed. A Chinese speaker (the author) would not have used the word "装机" if he meant IL76. In other words, in Chinese terminology IL76 is "not" categorized as an aircraft but a test-rig in the same way as a high-altitude test stand.
Of course, nobody could be always correct in language/wording, but that is the convention. The issue IMO is a lost-in-translation "testbed has no literal equivalent word in Chinese".
The Il-76 test rig is a flying test rig, "飞行试验台". Mounting on a test rig would be "上台", whether it is a ground test stand or a flying one.