Since the other reply was unhelpful: apps are supposed to have limited privileges and isolation from each other, yes... But the whole point of malware like this is that they figure out ways to break those restrictions and get escalated privileged.
You can get more technical detail from reading the report, in this case it looks like the app does not contain malware, but instead requests an update after install that contains the bad code and then breaks the app limitations and scans for the target banking applications and copies the security certificates.
It looks like they are doing it after app install with a malicious patch. This patch asks for SMS and accessibility access to gain privileges necessary to get into the banking apps. I haven't thoroughly read it but just looking at the attack chain that's what I gleaned.
He's being condescending because he believes as a developer nothing is actually fully secure. If I spend 100 hours building and securing something, that's not going to stack up very favorably vs the 1,000's or even 1,000,000's of hours attackers and communities can spend trying to break my security layers.
Basically, he's a dick in how he answered the question, but the truth every software engineer learns, is that there is no fully secure system. There's always an angle/attack vector you didn't think of and secure.
Android as a system has too many moving parts. You not only have to worry about various device manufacturers compiling their own versions of AOSP, you have to worry about how manufacturers package unremovable apps like facebook, candy crush, etc.
The backdoor is actually the front door.. and it is app vendors who are actually the customers... not the phone owners.
The main reason smartphones took off is that business people were salivating at an always on, always listening device with 10+ sensors collecting data on this whole world. And we pay for the privilege.
Android has to be designed to collect data and show you ads. Is it really surprising that security here is just security against free access to this data from outsiders.. and not caring about your security?
There's no such thing as perfect security.. unless your application is trivial and doesn't do very much. Android is designed to collect data from the dozen plus sensors on your phone in order to get money from app vendors to push ads.
Yes, the app doesn't steal any information from other apps. The report says the malware just displays a fake bank login page, in the hope the user gives it their details willingly.