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This is addressed in the second paragraph of TFA:

"Stuff which is somehow limited (stack overflows, arbitrary configuration, etc) is still considered Turing complete, since all "physical" Turing machines are resource limited."

In my opinion, worrying about infinite memory, in regards to Turing completeness, makes the task of implementing computation much less interesting.

Also, I'm pretty sure CSS only does one generation (or a finite number of them) before stopping anyway.