Interesting read from Seachem. About 3/4 of the way through the article, they provide an explaination of why freshly mixed s/w contains ammonia:
“A silent contaminant of commercial salt is ammonia, arising from the use of magnesium chloride as a principal source of magnesium.”
There’s also a good read on that forum whose name shall not be mentioned regarding ammonia as well as a quick “tutorial” on test kits. Overall (and that includes my humble opinion), it sounds like you might want to look elsewhere.
You may just be noticing this now because you’re now looking for a cause for the ammonia in you tank. And while the salt mix having ammonia in it at the start may contribute to your ammonia readings, they SHOULD be reducing over time, particularly if you delay water changes for a while.
I’d let the system stable out (with no water changes) over a period (month?) and see if the bio-filter can knock some of the reading down. If it does not, then you are overloaded for the amount of bio-filter you have. (Use the time you’re freeing up by not doing water changes to educate yourself on the differences between Nessler type test kits and salicylate type test kits.)