Nope to what? Are you saying that binary diffing is possible or that the amount of effort required is (remotely) comparable with analyzing source code?
I would like to see evidence supporting the latter statement, if this is what you are saying.
Consider all the people who study closed source browsers (MSIE) and plugins (Flash) to write malware. Consider all the people who reverse engineer malware to write protections or ransomware decryptors.
The people who can do such work don't work exclusively for the NSA and Google, and you can probably hire them for $1000 a day. but none of them will do tricks for you for free just to prove that they exist. They're too busy making money.
I saw some of the work described in this [1] excellent paper on reverse engineering NSA's crypto backdoor in Juniper equipment being done live on twitter. People exchanging small pieces of code, piecing together all the changes that were made in order to allow passively decrypting VPN traffic.
At one of the better-attended Black Hat USA talks last year, a team from Azimuth got up and stage and walked the audience through an IDA reverse of the iOS Secure Enclave firmware. Your argument is that it's somehow harder to reverse a simple iOS application?
No, you are oversimplifying the problem a lot.
In an Open Source project it is possible to create transparency in the development process by every commit public and allowing 3rd parties to mirror the sources repositories as well as perform reproducible builds, sign the artifacts and so on.
Once a project has been reviewed, it becomes pretty difficult to sneak in a backdoor later or deliver a backdoored build only to some specific targets.
In case of closed source smartphone applications it's very Hard to reverse engineer every single release simply because it takes a staggering amount of work.
It's also Hard to verify that some unsuspecting users are receiving a "custom" apk and block the update automatically.