Yes, that is the case. The X is always pronounced as a 'sh' sound. The same goes for most if not all Mayan languages as well. In general, Nahuatl is written phonetically using the Latin alphabet. There are no hidden sounds or sounds that change depending on associated letters as there are in other languages written with a Latin alphabet (i.e. many words in English, or the pronunciation of 'c' in Italian). Attempts have been make to introduce even more phonetic spelling changes, such as writing 'hua' as 'wa', but these changes are not widely adopted. Instead, most scholars continue to use the colonial Spanish way of spelling words with the exception of no longer using colonial letters like ç or ſ to spell out words. So seeing the deity name Tlahuizcalpantecuhtli, 'Lord of the Dawn', one should able to reasonably sound it out once you know which sounds are associated with which letters - [tla-weetz-cal-pan-tay-coot-lee].