My answer has more to do with your side question than your main one (which others have addressed) but I think a look at the definitions and current usage is relevant.
MW has "linguistic" as
of or relating to language or linguistics;
a "linguist" as
a person accomplished in languages; especially : one who speaks
a person who specializes in linguistics;
and "linguistics" as
the study of human speech including the units, nature, structure, and
modification of language
MW goes on to say of "linguistics":
Study of the nature and structure of language. It traditionally
encompasses semantics, syntax, and phonology.
With the rise of historical linguistics in the 19th century,
linguistics became a science.
Someone with a strong propensity for studying/learning foreign languages may be said to have good linguistic skills, but somebody who studies how a language or languages in general work OR who is accomplished in languages, is a linguist.
At first sight, the definition in your question, "A person skilled in foreign languages." surprised me as it seemed so simplistic (compare to MW's definition above), but looking at the example sentences that accompany this definition, we have
He was also an accomplished linguist speaking nine foreign languages
including Chinese and Tibetan.
He was a formidable linguist, speaking 25 languages and many more
A skilled linguist, Marianne used her new-found freedom to become an
interpreter for the British Army.
In each of these instances, the person described as a "linguist" is accomplished at speaking (generally a great number) of foreign languages. These are not people who enjoy studying foreign languages, or have only learnt keywords/phrases, or simply have a knack for picking up languages - these are people who likely will have studied these languages in depth and arrived at a high level of fluency. These people require "linguistic" skills, but more than likely have also engaged in "linguistics", and thus are "linguists" in the "true" sense of the word.
Of course, anybody can use "linguist" to describe themselves, but this does not mean that they are "linguists" from the field of "linguistics".
It pays (for the YouTube ranters) to keep in mind that etymology is not a dictatorial force, and that it is current usage that dictates current meaning. There does seem to be a plethora of things online these days about how much everyone loves grammar and how important it is (examples) - a sort of "pop-linguistics" trend that has perhaps surpassed that of the "pop-psychology" that was everywhere a couple of decades ago. People are free to describe themselves as "linguists" if they have an interest or any knowledge of language(s) at all. It is a matter of preference how freely one wishes to apply the word, but it pays to keep in mind how your audience is likely to understand the word.