It is correct to say that Lynn Hejinian says that there are multiple readings of an "open text". However, she does not say that all readings would be equally viable or valid. There are two passages in her essay that address this. First, she writes (emphasis added),
though the “story” and “tone” of such works may be interpreted differently by different readers, nonetheless the readings differ within definite limits. While word strings are permissive, they do not license a free-for-all.
Further down, she approvingly quotes a passage from Umberto Eco's The Role of the Reader (emphasis added):
I call these interpretative moves inferential walks: they are not mere whimsical initiatives on the part of the reader, but are elicited by discursive structures and foreseen by the whole textual strategy as indispensable components of the construction.
In other words, due to a text's "discursive strategies" some readings of a text will be more convincing than others. The text will more strongly "resist" interpretations that work against it. For this reason, not all readings will be equally viable.
This is not an unusual position. See for example chapter 4 ("Interpretation") in Terry Eagleton's book How to Read Literature (Yale University Press, 2013):
Literary works may best be seen not as texts with a fixed sense, but as matrices capable of generating a whole range of possible meanings. They do not so much contain meaning as produce it.
However, this does not imply that "anything goes". Eagleton later says,
There is no single correct interpretation of 'Baa Baa Black Sheep' [a nursery rhyme that Eagleton had discussed earlier in the book], or for that matter of any other literary work. Even so, there are more and less plausible ways of making sense of it. A persuasive reading must take account of the textual evidence, though establishing this evidence itself involves interpretation.
Note that Eagleton, unlike Lynn Hejinian, does not exclude "some contemporary lyric poetry" and "detective fiction", which Hejinian categorises as "closed" types of texts. From the point of view of modern literary theory, Hejinian's category of "closed texts" comes across as a strawman, since theory would not reduce, for example, the meaning of a detective story to just the revelation that the butler did it with the candle stick in the conservatory. Meaning is much more than that.
Note: This is a response to the version of the question from 14.01.2019 - 02.08.2020, which asked, "I just wondered if each different method, way, of reading a text is equally viable." The words "if they are at all viable" were added after this answer was submitted.