I don’t know whether Krashen used the word “method” himself, but the way people talk about it in this forum or in language related YouTube videos who are “pro Krashen” is that they state they learn it based on his ideas. So at least they seem to consider it a method, whether or not they used that word.
What would be the opposite? To not try to understand the sentence?! You summary perfectly sums up what Krashen is about in my perception. A set of vague and obvious statements.
Of course I choose material that is, within the context of my learning approach, at a difficulty level that provides the best learning efficiency. That isn’t a great idea. What would be great would be a measure that allow us to judge reliable on what material will yield the best results for us. Just stating i+1 isn’t really helpful. Not to mention that it hurts me as a mathematician how he butchers algebra.
Of course my emotional situation effects my learning efficiency. But which emotional situation is best for learning? Should I be in the same mood all the time anyway, or should I learn under different emotional situations? Is it the same for everyone? Is it age-dependent? And how does the reason why I learn a language come into play? If I am learning a language to work in a foreign country, should I learn under more stress as I may be stressed there, too, and this might make sure I can access that knowledge than?
What does his natural order of grammar acquisition and his statement that people learn grammar via input (an idea that dates back to the ancient Greeks and was prominent in the late 19th century) actually imply? If I am learning the grammar via input does this mean actively studying the grammar is pointless, or should I study only specific grammar? Does the natural order imply that if I try to study grammar out of order I will not be able to understand it? Is any of this actually proven or did Krashen ever invest any time in investigating this before he released his work?
I second what @LeifGoodwin already stated. I don’t think that his theory would withstand a scientific investigation. The main issue already lies already in it beeing way too unspecific in its assumptions, making it hard to do so. The first property any scientific theory needs to have is that it needs to be falsifyable. Otherwise it isn’t science, it’s religion.
I can see that people find Krashen appealing, especially if their memory of their language classes in school is mainly negative. But that doesn’t make it correct.