Argument doesn't hold water. Apple's other devices are 4K compatible. This is the only exception. If 4K is only for marketing purposes then Apple is marketing the hell out it. Heck, Apple's already gone past 4K into 5K and higher territory (rumored 8K).4K is kind of like Bluray was when it came out. Everybody marketed supporting it but Apple. Apple elected not to support it because licensing fees were high and the hit on hardware performance because of mandated DRM was high. That stuff never did get ironed out. Places like Redbox still largely rent DVDs not Blueray.
Here 4K is mostly just for marketing purposes. Although 4K TVs are common, available content is low. Most 4K content is compressed, and isn't much better than 1080 uncompressed.
More importantly, there are lots of technical hurdles. 4K uses significantly more data. Most people have data caps. Also there is speed issues. Netflix has some 4K compressed content, but my internet provider doesn't provide speeds good enough to receive the 1080 content at full quality much less 4K. Also there are hurdles related to wifi speeds.
Apple recognizes 4K as downloadable content is concerned is not yet there yet.
Bolded from your quote: There aren't a lot of technical hurdles to implementing 4K. Yes it uses more data and some people have caps. It's not an all or nothing proposition where everyone has to use 4K or no one can. For those who can, the option should be there. 4K video also consumes a lot of memory. That didn't stop Apple from adding that capability to a 16GB phone.