I'll just ignore the Reio, he just doesn't say anything useful to the topic.
About the translation, I had interpreted that it was about Mifune, but after reading what Qliphoth said about it being two sentences, I interpreted that the first sentence speaks the mechanics of the technique and the second uses Mifune as a comparison or something. In the case of Mifune, I interpreted that the phrase is saying that the technique combine with Mifune's sword and swordmanship, something like: "This attack combines with him". Using a different expression, it would be like you seeing a woman in a dress and saying, "That dress combines with her." Something like that, just saying the technique combines with Mifune.
In the case of Outlier, I don't think it would be one, the fact that Madara uses an LS attack doesn't imply that weaker characters can't use too, Madara himself even uses natural lightning in battle, it implies that characters bellow him can't use Raitons or other lightning speeds attacks? I do not think so. And there's no problem with the scale.
About scale, only Sasuke has a feat, and it was calculated and accepted at 0.13c, Relativistic only, and there are few who would scale, there is still Orochi's feat in the novel that can put characters like Konohamaru in Sub-relativistic. And the characters that scale with this feat are the ones currently in Sub-relativistic, it wouldn't be such a huge upgreade in the scale.