Gavin Menzies' books are entertaining but considered to be fantasy even by Chinese scholars.
Some academic (a sinologist) told me that Menzies is not well respected in academic circles.
Now, that's not necessarily a good criterion to go by (any group of people, including academics, can be closed-minded about a particular topic - especially if it's their 'pet topic'), but in this case, I think that many sinologists (being interested in and positive towards Chinese culture) would welcome
news about China being advanced in a particular aspect of history which hadn't previously been recognized.
But they don't. They think it's just sensationalist stuff, without much serious research. If I remember correctly, the author doesn't even read or write Chinese.
I put myself in a similar position. Being self-identified "Chinese" I am 'proud' (hopefully in a non-chauvinistic way) of the long cultural continuity of Chinese civilization, and of its amazing contributions to human culture, many of them at such an early stage of history. I would certainly be quite happy if it were really true that Zheng He reached America at such an early stage. But this has to be based on fact, not on just a desire to see more 'achievements' added to the story of Chinese history.