I haven't read the book, but it seems to me that the point is that certain advancements in
computation are universal, and previous industries can be considered as serial advancements in
(or facilitators of) general computational capabilities. So it doesn't matter that
advancements in steam engines were finite; rather that subsequent advancements made possible
by steam engines, no matter the material industry, had the effect of in kind furthering
[exponential] growth in computational capabilities in general.
Theoretical computational advancement can continually progress as long as materials science,
no matter how disjoint, continually provides sufficient capabilities for the realization of
the next rung on the theoretical ladder.
The overall argument is not particularly persuasive, I agree, but not obviously fallacious.