"means they are one consonant" is the perspective of the language's speakers, not of the actual sounds they are making. The former is a subjective matter, the latter is essentially objective. Whether speakers believe they are saying one consonant or two, a "ts" affricate always consists of stopping the air with the the tongue behind the teeth (the t) and then releasing it with a lot of turbulence with the tongue still behind the teeth (the s). Those sounds are physically distinct, even if speakers think of them as comprising a single logical unit.
This is also consistent with there being a slightly differently pronounced sequence comprising a t and an s sound which is not analysed as a single consonant! Both are two physical sounds.
In my language most native speakers don't realise that diphthongs are made of two vowel sounds, but they still are :)