Is it possible that a iphone can be hacked remotely because this app claims it

If you read it carefully, it requires you to have their Apple ID and password, and as Johnathan says, if you don't share that, they cannot use it. It isn't an app, and it isn't a hack, it uses this company's servers to log through the user's Apple ID and password. Stay away from things like that. You don't want to be sharing this information with 3rd parties.

Their final claim is both right and wrong.

"all you have to do is gain access to the target user's iCloud credentials and you can hack their phone"

If you have their credentials, you can cleanly access their account. No hack required. You are them.

There is no way to remotely access their iPhone, even with credentials. It must be in your possession.

If I have someone's Apple ID and password, I don't need any software to hack anything.

I can already access anything.

The answer is, dont give out your Apple ID and password.

No, it isn't. I cannot seem to find that app in the US App Store. Where are you seeing this, in another country?