This is incorrect. Old Necrons represent a theme known as Man vs Machine. While Tyranids represent the theme known as Man vs Monster. The differences are quite stark, actually.
Man Vs. Machine deals with an antagonist that could very likely be something humanity has made, has the capacity to make, or that something else made. Regardless of the origin, the result is still that man vs machine features an antagonist that is unyielding, emotionless, unliving, and that has the single-minded intent to murder the protagonist(s). Examples include Terminator 1, the T-1000 from Terminator 2, and many of Isaac Asimov's great science fiction works.
Man Vs. Monster portrays the antagonist as much more of a 'natural threat'. Such as a beast, or living creature running on the instinct to survive. It's goals may be simply to feed, or reproduce, as opposed to simply 'murder everything in sight'. Or it's goals may be infinitely darker than simple biological drive to survive. In any case, the man vs monster taps into the very primal fear of a living creature that may act in ways that cannot be predicted. Much like a tiger, crocodile, tyrannosaurus rex, or the Xenomorphs from the Alien movies.
Therefor to say that the Old-Crons are just 'metal Tyrannids', is grossly inaccurate. Much in the same was as saying that the xenomorphs from the Alien movies are just 'shiny black Cylons'.