I would struggle to find a teacher who wouldn't agree that development of concentration and insight (along with other aspects of the path) are not linked. Do such teachers actually exist in real life? In my experience, what I see is emphasis on different aspects at different stages of development. And even the Buddha, in his "gradual training" suttas is emphasising different aspects at different stages: viewtopic.php?f=11&t=14478&p=211828#p211827
and talks about getting instruction in different aspects: viewtopic.php?f=11&t=14478&p=211828#p211724
As Ajahn Brahm says, in Mindfulness, Bliss, and Beyond, Page 25 [after describing how to attain deep Jhanas, using
instructions, including the breath nimittas, that are reminiscent of the classical descriptions, as taught, for example, by Pa Auk Sayadaw, in "Knowing and Seeing"].
For those who are misled to conceive of all the instructions offered
here as “just samatha practice” (calming) without regard to vipassan›
(insight), please know that this is neither vipassana nor samatha. It is called
bh›van› (mental development). This method was taught by the Buddha
(AN IV,125–27; MN 151,13–19) and repeated in the forest tradition of
Northeast Thailand, with which my teacher, Ven. Ajahn Chah, was asso-
ciated. Ajahn Chah often said that samatha and vipassana cannot be sep-
arated, nor can the pair be developed apart from right view, right
thought, right moral conduct, and so forth. Samatha and vipassana, Ajahn
Chah said, are like two sides of one hand. In the original Buddhist tra-
dition they are inseparable. Indeed, to make progress in the seven stages
of meditation I have described, the meditator needs an understanding
and acceptance of the Buddha’s teachings, and one’s virtue must be pure.
I would also note that nany of Ajahn Chah students (Ajahn Tiradhammo, for example, who was in my country for a number of years) are not so enthusiastic about developing deep jhana as Ajahn Brahm is, feeling that developing "just enough" concentration allows faster progress.
We are fortunate to have access to a variety of experienced and talented teachers of the Buddha Dhamma. In my opinion, that they teach different approaches (and have different interpretations of "jhana" for that matter) is a reflection on their particular experience of what works for them and their students, not a result of some failure by some "sides" to understand the Buddha's teachings. It would be rather tragic if that were, indeed, the case...