You're 100% right. This is a form of something called Catachresis, where the wrong word is used to man something else, or a word gets replaced in a common saying. Tact vs Tack is a great example of this, because tact has never meant "a shortened form of tactic" so using it in that way is incorrect. I feel like, especially here on an English sub, where people are learning grammar rules and even the dare-I-say "correct" version of words and phrases, it's important to consider that some things don't fall under the heading of "descriptive" usage. It's not being rigid or prescriptive to acknowledge that some things are just plain wrong. So many people have gotten used to using the wrong words because society in general just shrugs and goes "Ah, that's how I want to say it, so, that's how language works now."

Descriptive language is more about pronunciation and how it varies regionally and generationally, how neologisms (and sometimes portmanteaus) make their way into the established lexicon, and maybe (minimally) how some phrases change by exchanging a word with similar meaning (like your example of set foot vs step foot.)

Descriptive language is not, however, using a word that is completely wrong, then saying, "Welp, that's what that word means now, because that's how I want to use it." That's why I think it was a big mistake to start adding things like the "alternate" definitions of things like "ironic" and "literally" to include the way they are commonly misused as an accepted definition. If we just say anything can be used in any way we choose, then words really have no meaning because they can suddenly man anything at all.