τῐς is not the Koine equivalent of the indefinite article. Koine does not have an indefinite article; often the indefinite semantic force is connoted simply by the lack of a definite article. τῐς is an indefinite pronoun.
Here is Mounce, Basics of Biblical Greek:
I mainly agree, except that τις can ina secondary sense function just like an article even though it is a pronoun: hence τις ἀνὴρ in Acts 3:2.
Here is what Caragounis in John 1:1 has to say further on the article.
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"Because the Greek article originally (in Homeros, etc.) was a demonstrative
pronoun, which later came to lose much of its demonstrative
force, and to that extent assume the meaning of the article—especially in
Platon's writings, where the article achieved its richest and most varied
uses—it cannot be compared to nor can its uses be determined by the
way in which the English article is applied. If we want to understand the
ways in which the Greek article is used, it is important that we disregard
the uses of the English, German, etc. articles, and that we study the Greek
article against the background of its own usage in Greek literature. This
does not imply that there are no parallel uses of the article between Greek
and English, and German, etc., but that methodologically it is better to
dispense with reliance on the English, etc. article for determining the
meanings of the Greek article.
For example, whereas English would use the anarthrous "Man" or "Humanity"
to indicate class, the Greek would use the arthrous ό άνθρωπος
or ή άνθρωπότης to express the same idea. In other words, the uses of
the English article do not coincide with those of the Greek article, and
we would do well not to impose on NT articular or non-articular constructions
ideas based on the uses of the English article. Thus, when a
Hellene says ό άνθρωπος, the construction is not definite in the sense
that he is speaking of a particular man, but generic: through the use of
the article, he concretizes all men (i.e. humanity) in the arthrous singular
as representative of the entire class of men. In saying ό άνθρωπος the
Hellene thinks of all that belongs to the category of "man", but not of
beasts, etc., that which distinguishes, demarcates, and defines man from
all other categories of creatures, that which belongs to the concept Man,39.
At the same time, the entire group of men (i.e. the whole of humanity) is
thought of as a concrete whole. Thus, ό άνθρωπος θνητός έστιν means
"all human beings [without exception] are mortal". The so-called indefinite
form άνθρωπος τις means "someone of the genus man", in other words, it
describes man as a substance limited, by itself, and as indefinite: "a certain
man". Thus, too, the abstract "man" by receiving the article becomes
concrete or definite: ό άνθρωπος δς ήλθεν έξ 'Αθηνών "the (particular)
man who came from Athens".
Many times the article is used in connection with a person that has
been mentioned before: Acts 4,22: ετών γάρ ήν ... ό άνθρωπος (cf. Acts
3,2, where the same person is described as τις άνηρ). Even though we
translate "the man was ..." in English the force is "i/iar man [of whom I
wrote earlier] was ..." In this capacity the article is used in its original
demonstrative force, rather than in its pure articular sense. But when the
subject is presented as a general concept, without any specification, classification,
etc., it is anarthrous. Thus Platon, Theaitetos 152 a: πάντιυν
χρημάτων με'τρον άνθρωπος means not so much "a man is the measure
of all things" but "anyone who is a man [i.e. who shares in all that makes
up a human being] is the measure of all things". From this it is a small
step to the question of the arthrous or anarthrous predicate.
The predicate is usually anarthrous, because it does not denote a
definite person or kind or class but only property or essence, which is
predicated of the subject."