You say 'likely' but you're talking about an a priori belief. Post-biological civilization is purely speculative science-fiction at this point, there is not a single real example that we can point to yet. This might one day change if we become a post-biological civilization, but so far not a single example of such has ever been seen.
Based on what we've already empirically observed, any life out there is likely to be biological because that's the only sort of life we know can exist. Furthermore, we're unlikely to ever encounter life outside our solar system anyway. We've been looking for decades and can't find anything. The techno-signatures we speculate advanced civilizations might produce remain elusive after years of searching with increasingly capable instruments. Primitive life is unlikely to ever produce conclusive signals we can detect; the best we'll get is suggestive but inconclusive atmospheric analysis which we'll be incapable of investigating properly because going there for a closer look is technologically and socially impossible.
My guess is we're not alone, but we might as well be.