This is all bullshit. Each company pays what is profitable for its business model. Work in a hedge fund? Your pay is commensurate with the business model. Work in a digital agency, likewise. As far as I can tell a digital agency is a digital agency, is a digital agency. They all work the same market with the same overheads. It doesn't matter that the dev in the hedge fund is doing precisely the same job (in my hypothetical scenario).

"You are paid a little better than someone with your same job in your geographical area."

Of course you are, otherwise your employer wouldn't be in business. The operative word being "little". Substitute "a lot" and either they have a disruptive business model and are willing to pass on the cream to their employees (unlikely), or they are going out of business.

Give me any dev in any city in my country and I will be able to tell you what they are on based on their seniority and sector plus or minus a few grand (and what's a few grand after higher-rate tax?).

Tldr - this policy is moot because salaries are open secrets.

FWIW, if you want to earn more go into contracting (i.e. take on more personal risk), move sector (i.e. go into finance), start your own company, or move into a more traditional profession.