Symmetrical output impedance simply means equal current strength of N and P output transistors, which in practice means about doubled W/L for P transistors. You'll notice, that the output characteristic of digital logic devices isn't tightly specified. If you review vendor SPICE models, you'll find NOMI, FAST and SLOW parameter sets. Unfortunately, the process variations aren't necessarily identical for N and P diffusion steps. As a result, symmetrical output impedance doesn't mean more than symmetry for a device with typical parameters. There will be still an impedance variation.
Nevertheless using HC single gate buffers or inverters for analog purposes, e.g. drivers with defined output impedance can work, if the drivers share of the total inpedance isn't too large.