The traditional Spanish alphabet has no K or W, but has Ch, Ll, Ñ and Rr as separate letters.
Recent (in the last 50 years) decisions by the Royal Academy of the Spanish Language have permitted the use of words with W and K, and reduced the guidances about Ch, Ll and Rr. Now, for example, in most places words beginning with "Ch" are sorted in the order of Cg, Ch, Ci rather than Cz, Ch.
The IFT can't even get its sorting straight. The TV tables go Camp-Chih-Chis-Coah but the AM and FM tables use the "traditional" sorting and put Chihuahua and Chiapas after Colima.
As to K and W in Mexican callsigns, those have been used forever (and even were desirable — the backstory of XEW, after all, was that they wanted to be confused with an American station). XEK, originally in Mexico City, got its calls in the 30s.
I also noticed from that list that there are several new stations getting six-letter call signs. I know Mexico had some five letter call signs, such as XETRA. And Canada gives out some five-letter call signs to CBC stations to indicate whether they are English or French, or are secondary stations getting most of their programming from a big city CBC station. But that's only for CBC stations. Privately owned stations are all four call letters.
Six-letter Mexican stations have existed since the late 80s. I believe the first might have been XEVILL-AM Villahermosa, which is now XHVILL-FM and was awarded on December 13, 1988. A raft of others were allotted in the 90s to the INI (now the CDI) indigenous radio system (XEETCH, XEXPUJ, XECOPA, etc.). A few 6-letter calls have been created by callsign changes, such as XHFAMA and XHCORO (in 2000 and 1996, respectively, while they were on AM).
The first TV with a 6-letter callsign was XHUNAM, which has primarily operated as an experimental digital station but may be gearing up for a full-power conversion at some point. It was permitted on November 27, 2000. There are now many more 6-letter TVs because the first 26 SPR transmitters (XHOP**,
Organismo
Promotor de Medios Audiovisuales) and all Imagen TV transmitters (XHCT**,
Cadena
Tres I, S.A. de C.V.) have 6-letter calls.
6-letter calls usually mean the concessionaire has selected their calls. The Veracruz Social Wolfpack (my moniker for a group of interrelated permitholders and concessionaires for noncommercial radio stations in Veracruz with the blandest "cultural" names possible and connections to the press office of embattled ex-Governor Javier Duarte) holds *five* FM stations with 6-letter calls, so XHTRES is no surprise (also: XHPAPA, XHALAM, XHTLAC, XHSANR).
There are also some 6-letter calls that were created in AM-FM migration, like XHERIO, XHEVAB, XHESCC and XHROOC. Each of these was five (or four, in the case of XESC) characters prior to migration).
