Having the register specs and behavior of the various blocks inside the chip is a lot like having the API of a library. It lets you use it, but doesn't tell you how it's implemented internally, which is where most of the secret sauce happens. This register/spec is what a lot of people like me wish for.
There are two levels of "open source" for hardware: the first is where the registers and behavior are documented, allowing programmers to write their own drivers for the hardware, but for moderately complex stuff doesn't get you far for copying it. The second level is where the implementation details of the chip (the Verilog or VHDL description of its behavior) are open-sourced. Only the latter would let you "easily" create a knockoff, and even that isn't within reach of any household hacker, as the base cost of fabbing a chip is still in the hundreds of thousands of dollars, but yeah that would be within reach of an undercutting competitor.
(there's even a third layer of open source where the Verilog/VHDL is turned into the final layout tuned for the fab, think of it like having the source-code vs the final assembly for a given architecture. That step is costly too)
There are two levels of "open source" for hardware: the first is where the registers and behavior are documented, allowing programmers to write their own drivers for the hardware, but for moderately complex stuff doesn't get you far for copying it. The second level is where the implementation details of the chip (the Verilog or VHDL description of its behavior) are open-sourced. Only the latter would let you "easily" create a knockoff, and even that isn't within reach of any household hacker, as the base cost of fabbing a chip is still in the hundreds of thousands of dollars, but yeah that would be within reach of an undercutting competitor.
(there's even a third layer of open source where the Verilog/VHDL is turned into the final layout tuned for the fab, think of it like having the source-code vs the final assembly for a given architecture. That step is costly too)