DIY PC retailers?

[Encrypted11]

Having said that, it is said that the 6 core turbo may or may not hit 3.8 Ghz on the i5 8400 since Intel did not officially state the 6 core max turbo speed.

Not true.. Open Intel XTU and Intel Management Engine driver (its a tool that can access the big brother in the CPU ).
These are values baked into the chip, your old sandybridge i5-2400 also works the same way.

And how did AnandTech find out about this without Intel's technical marketing docs?

Oh by owning the CPU physically and use XTU that reads the CPU backdoor (Management Engine)

Spoiler

e.g. i5-3230M's non-AVX turbo multiplier table

This is a marketing dept change, but how the CPUs work hasn't changed.. Tech tubers like AdoredTV have unfortunately spread FUD saying he won't be surprised if the retail chips won't boost to their baked in non-AVX turbo values.

But has yet to provide proof on a Intel confidential chip vs. a retail chip has a different turbo table built-in. If his FUD is real and proven correct, such a shady move would be class action worthy material in the states already.