It’s complicated, and I’m sure others here can supply more detail.
For me, lineage should imply credentials based on a solid foundation of proven methodology/practice in whatever martial art, and responsible teaching of that art. In as such as the lineage is the tradition of the art, it can be said to be the fabric of the art. For, without able practitioners, the art would be on paper only, if that much.
I think of the above as reasonable. It leads to expectations of being taught properly, according to the ways of the past, and possibly with the expectation that each student with sincere interest and a good heart will be strengthened and led to succeed either to master that traditional system, or to learn to the extent of his/her abilities (which might include advanced work). However,
that a teacher is of a respected lineage, and acknowledged as such, does not guarantee the above. It just lets you know that a teacher is part of the family tree and has knowledge as it was passed down, and practices it. It doesn’t mean that the current training is identical to, or comes close to that received by the teacher, or, that different teachers from the same lineage are equivalent in terms of ability, or other individual traits. People are people.
So, for instance, if a student wants to learn Eagle Claw, he’d be advised to get as close to the source as possible, and that’s where lineage comes in. When a modern, composite art is put together, knowing the teacher’s roots is also advisable. What you have then is a hybrid of sorts, which either succeeds or doesn’t. I don’t think of this as being faulty or suspicious, except if deception is involved.
The issue of questionable lineages, especially in terms of modern day practitioners and those of recent past, gets on my nerves. If a person is flat out lying, I have no use for him. It is likely that there will be other dishonesty, and a greed for recognition or economic gains that is harmful to all. That is one reason to steer clear of this kind of suspicious situation, in my view.
If the history of a system as a whole, or a supposed offshoot of a martial system, appeared to be on quicksand, would you step in the quicksand? You might research to see if critics have a case, and why.
As for who can or cannot defend themselves, well, the proofs become easy. Please note that I did not read the thread you referred to before writing.
Cody