Writing a letter can definitely signal a greater amount of attention being put into an effort. But can’t it also be used as a medium to lie about your level of affection and mislead just as you suggest emojis are?
While I think the article does take the position to its extreme on the sense of ignoring medium, this response seems equally extreme to not consider that the medium should really match the expectations of the audience. For some people and under certain circumstances, the digital option is probably more appropriate and sometimes the physical is more appropriate.
However, the author’s main point seems to be that the digital route is often summarily rejected as impure and is not given the respect it deserves as a justifiable medium for communication.