I would guess the implication is that almost all phones are built in China. So "Chinese cellphones" is a curious term - "Cellphones designed by Chinese firms" and "Cellphones manufactured in China" are wildly different circles in the venn diagram of mobiles.
Only Chinese cellphone design companies have a meaningful direct responsibily for Chinese government, which makes the difference. Manufacturers have resposibility too, but if they manufacture something else than intended, it will stop quite quickly.
> What is the difference? To not buy branded Chinese phones seems just protectionism and has little to do with security.
How does "protectionism" have anything to do with this? It's weird to inject that economic framing into a question about what kind of policies you want to be subject to, and who you want to be setting those policies.
I have nothing against wanting to protect and industry, but it should not be done with misconceptions.