No, I wouldn't say that. Those filters make a huge difference to the compression ratio!
The png optimizers you see (optipng, pngcrush, etc)? What they do is carefully pick the optimum filters to get the best file size. So I'd say those filters were the most important part of the compression.
You are correct. I almost qualified the statement as such, but figured "pretty much" was good enough to get my point across in a hurry. My point being that zlib compressed 24-bit RGB values in a PNG is normal. It's a different approach than lossy DCT quantization used in JPEG.