it would still have sign if and only if it would still be a nuclear arms state, given the fact that not 20 years before 1970 china was threaten with nuclear destruction by both superpowers, and in 1970 china was still against both super powers.
reasons are easily given see above for a very simple one of being threatened openly by 2 superpowers, thus its for defense only especially given that china has the NFU policy in place
the thinking does not apply to india because essentially India was not threaten with nuclear destruction by anyone, rather it brought nukes to SE Asia prompting Pakistan to develop its own nukes
if india had its test before 1970 the results would be the same, it would depend if india is recognized as a nuclear weapons state, if the world granted india that status tomorrow it would probably sign the NPT soon