The question that is answered in the link is not identically to the answer i seek here:
"Canon law requires some “form” (or “format” for marriage) (canons 1057; 1128, §2; 1161, §2; 1162). For a marriage involving one or two Catholics, this means the requirement of canonical form – i.e. that the consent be exchanged before a priest or deacon and two witnesses. For marriage involving two non-Cahtolics, the “form” can be any public form. (“Public” in a canonical sense means that it can be proven through documents or witnesses, not that it is announced in the newspaper.)
This results in the conclusion that a common law marriage is not valid in the eyes of God and his Church because of the “lack of form.”"
The writer simply concludes that common-law marriage is not valid. He can do so, because
en.wikipedia.org/wiki/Common-law_marriage#Essential_distinctions
“Common-law marriages are not solemnized before witnesses in a wedding ceremony.” and therefore cannot meet the public criteria.
I made the mistake of not being aware that common-law marriage has a distinctive meaning especially in leagl terms, otherwise i should have corrected it immidieately. What i describe in the OP is not a common-law marriage according to the usual e.g. wiki definition, because
"They fall in love with each other and promise each other to be faithful till the end and have some children. They inform their relatives, friends and so on about their decision. "
There are promises exchanged and witnesses can testify to that.
And its made public:
“They move together and hang some sign at their entrance”
Therefore what A and B have is not common-law marriage according to wiki definition.
And the argument from the EWTN - lack of public form - cannot be applied to OP, A and B make it known they are married. They just do not bother to tell the state about it.
One understands rules, if one can apply them to different circumstances.
The other way round, if one applies them to different circumstances and fails, one has yet not understood (I assume my conclusion that the church would consider all traditional african marriages invalid, as they are often undocumented and just declared verbally to relatives and people known, to be false, because it sounds like a wrong conclusion).