How can a machine with fixed output for all inputs be considered a Turing machine?
There's no notion of &quot;can be considered to be a Turing machine&quot;.  Something either is a Turing machine, or it isn't.  There's a formal mathematical definition of what a Turing machine is: https://en.wikipedia.org/wiki/Turing_machine#Formal_definition
I never claimed that just because some process always produces the same output, that process must be a Turing machine.
There might be some confusion about Turing machines vs DFAs.  A computer could be modelled as a DFA (if we treat its memory size as fixed), or it could be modelled as a Turing machine (if we imagine that it's possible to gain more memory; or we just don't care to make use of the fact that memory size is fixed).  These two different ways to think about a computer have different advantages and disadvantages.  I suggest reading Is a PDA as powerful as a CPU? and How many gates until Turing Complete and How does an automaton model a computer or something else? and How does an automaton actually &quot;compute&quot;?, as I suspect they get at the core issue you are struggling with.