This could also mean "he is hairier than a dog" (because there is an aspirate mutation after "na", so "ci" becomes "chi").
There is a difference between hi and chi when spoken by natives, isn't there?
I'm not a native, but yes, I'm sure of it. The "h" is the same as in English, and a sound sample for the "ch" can be heard in the Wikipedia article on "voiceless uvular fricative", which also lists a bunch of examples in other languages, including German.
I have to say, with the typical cheap builtin speakers in some devices, it's possible at times for the various unvoiced fricatives to be rather harder to distinguish than they are when listening to someone in person. By "unvoiced fricatives" in the context of Welsh I mean the sounds written "ch", "ff" (or "ph"), "h", "ll", "s", "si" (as in "siop") and "th". Obviously I'm not saying they all merge, but for example "ll" and "si" can sound quite similar on mediocre sound equipment.