Releasing source code to paying customer is not "open source", it's just a clause in a contract. Microsoft allow some larger companies can get their hands on the Windows source code. Is Windows "open source"? I wish people would stop throwing around "open source" as a buzzword. IT'S NOT FUCKING OPEN SOURCE.
"the whole source code would be released as open-source to our paying customers."
The "as open-source" part there seems not entirely true. It's not truly "open" if only paying customers have a copy. Just "would be released..." would suffice, without the buzzword compliance thrown in.
Trivial is not necessarily the opposite of difficult though. Solving an easy but non-trivial problem can be done with average engineers, if you a sufficient number of them to get the work done.
This is what happens when a company hires genuinely exceptional people to do easy but non-trivial work: