I found this article rather... unsatisfactory. So the statute introduced the concept of "time immemorial". Except it sort of didn't, it just introduced a limit that later came to be referred to as "time immemorial". It also doesn't call it anything like "the barrier between that which we know and that which we don’t.", but indeed it does mark a point after which oral history is no longer enough. On page 100 in the linked translation:
> That in conveighing a Descent in a Writ of Right, none shall presume to declare of the Seisin of his Ancestor further, or beyond the Time of King Richard, Uncle to King Hentry, Father to the King that now is;
It then says the term "time immemorial" was actually introduced by the Prescription Act 1832 (and links to a different page than it is talking about) but that actually says:
> Whereas the expression “time immemorial, or time whereof the memory of man runneth not to the contrary,” is now by the Law of England in many cases considered to include and denote the whole period of time from the Reign of King Richard the First, whereby the title to matters that have been long enjoyed is sometimes defeated by shewing the commencement of such enjoyment, which is in many cases productive of inconvenience and injustice;
Perhaps I'm misunderstanding this since it was written almost 200 years ago, but this reads like it is describing an existing term in use in the Law of England. A term supposedly introduced at some point between 1275 and 1831 to describe the limit introduced by the Statute of Westminster 1275.
The included history is interesting, but how this date turned into the concept of "time immemorial" remains unknown.