Handwriting analysis cannot be used to exclusively and definitively argue for the authenticity, or otherwise, of a Zodiac communication. However, it can be used as a supporting argument where other evidence exists. This still hasn't stopped vociferous sections of the Zodiac community buying into the notion of an authenticated list of Zodiac communications based solely upon the narrative they have been sold from day one.
Let us look at the words of John Shimoda, who stated that the 1978 letter was "an attempt to duplicate Zodiac letters and is not authentic". The author of the 1978 letter was that good at duplicating Zodiac letters, they failed to duplicate the opening introduction of "This is the Zodiac speaking", which was always self-contained on a separate line in every confirmed Zodiac communication mailed to the San Francisco Chronicle, San Francisco Examiner and Los Angeles Times prior to 1978. The hoaxer had four examples of this to copy from the San Francisco Chronicle newspaper, when it released images of the October 13th 1969, November 8th 1969, April 20th 1970 and March 13th 1971 letters to the public (see images below). The hoaxer failed to copy any of these. Instead, they wrote "This is the Zodiac speaking I" (mimicking the unreleased image of the Melvin Belli letter and its punctuation error). The author of the 1978 letter had the January 29th 1974 Exorcist letter available to them - widely published in the San Francisco Chronicle on January 31st 1974 - yet they failed to misspell "truly" to "truley", clearly evident in the Exorcist letter. The author of the 1978 letter failed to add a customary running total, which would have been the easy option to choose for somebody wanting to convince us they were Zodiac. Instead they placed the word "guess". The author of the 1978 letter failed to keep the "This is the Zodiac speaking" introduction in line with the text beneath it (thanks Druzer). In the 1978 letter they used "indented writing" (to indent is to begin text with a blank space between it and the margin). This absence of indented writing during the introduction phase was present in every published letter up to 1978 (see below). So, you can see that the claim of the 1978 letter author attempting to "duplicate Zodiac letters" from the newspapers, carries no merit whatsoever. This individual wasn't attempting to copy anybody.
The Zodiac community have been eager to constantly enquire about the round of DNA testing reported in 2018, but show little interest in submitting the 1978 letter for familial DNA analysis. The reason for this apathy, is that many have looked at the 1978 communication and concluded it wasn't authored by the Zodiac Killer, primarily because they didn't like the handwriting or the tone of the letter. You cannot argue with research like this. For those who believe the 1978 letter is genuine, the search for Zodiac DNA is over.