How Does The Fake Irig Damage Your Idevice?

I personally don't see how it could. The signal from your guitar is analogue, and very faint. Fainter than you shouting into a microphone, which people do all the time I'm sure.

At worst a fake iRig could have a lot of signal noise, but I don't see how that could damage an iPhone, it would just sound bad.

Of course t can!!
an IRIG should cost a least $8/9 to consider it built with "decent" materials
They're offering it in Ebay for €10 including local postage (aprox. $1) plus the Ebay and Paypa charges. So he will end with about 7 euros...
WODERFULL stuff!! probably a kind soul that does it all for free.

Bad quality cheap connectors can cause damaged to the inner audio INPUT/OUTPUT jack.
It might not do it inmediatly but just give it enough time to corrode and gather oxide as it is made of with the lowest quality materials.
You don't have to be an engineer to know that for $4 you wouldn't even cver the printing of the counterfake box. Add to that the shipping from China and... just be cool!!
The cost of changing these woul set you bak a couple of hundrd USD.