You can take Ven. Eijo's word for it. He is a very good authority on all things Shingon. I have no idea why you assume there is any relationship between Shingon and Shugendo from my post quoted above or Seishin's.
I only thought that because of this sentence which Seishin wrote:
Shugendo was forced to belong to either Shingon or Tendai temples. However, I haven't got great knowledge of such matters.
I don't doubt that Shingon and Shugendo are completely or almost completely different in terms of their practices and philosophies. But if the Shugendo practitioners were forced to belong to Shingon temples, then this implies that they had some contact with Shingon Buddhists, and it is possible that they influenced one another in some ways.
I am just curious how much they influenced one another. Basically, I am interested in knowing what the relationship in Japan is between Shingon practitioners and Shugendo practitioners. Are they friendly to each other? Indifferent? Hostile? Do they ever do any practices together or do they always do their religious practices separately?
For example, in China, many people practice both Buddhism and Taoism. I just wondered if some people in Japan practice both Shingon and Shugendo.