I don't think so; I think nominal power at the drawbar tends to favour the diesels if anything. I think it was more complicated than that.
They thought too much in terms of a direct like-for-like replacement, and failed to anticipate the need for increased power in the general course of events, so the locos ended up being underpowered due to the change in requirements between setting the spec and bringing them into service. There was a desire to keep the weight down, too, particularly for the smaller types that would be expected to run on lesser lines.
They do seem to have failed to appreciate that a diesel has a hard limit on its power output, whereas a steam engine, with a large amount of energy stored in its boiler, can produce considerably more than its nominal maximum power for short periods; it can also be thrashed to produce excess power for longer periods, if the fireman can keep up, albeit with a rapid loss in efficiency.
They anticipated a lot more use of multiple working to compensate for lack of single large units, thinking this would be no real problem now that the additional loco didn't need an additional crew; but dis not 'appen (mostly).
Then overall there was the usual British paradigm of attempting simultaneous extravagance and parsimony, with the inevitable result that whether or not something was a good idea in the first place, the implementation is unsatisfactory due to important elements being underspecified, skimped or missing.
