这个句子是择比结构more A than B（与其说是B，不如说是A / 是A而不是B）的一种变体。

我们先看一个句子：

She is more an acquaintance than a friend.

= She's an acquaintance more than she is a friend.

与其说她是个朋友，不如说她是个熟人。/ 她不是朋友，只是个熟人。

这个句子变化一下：

She is not a friend, (she is) more an acquaintance. 她不是朋友，只是个熟人。

= She is not a friend, (she is) only/just an acquaintance.

词典对副词more这种用法的释义为：used to say that one way of describing a person or thing is better or more accurate than another.

你的句子就类似于 She is not a friend, (she is) more an acquaintance.这个句子。

British Land Art wasn’t about imposing on the landscape, more (about) a kind of landscape-orientated light conceptual art created passing through.

=British Land Art  was more about a kind of landscape-orientated light conceptual art created passing through than (about) imposing on the landscape,

英国陆地艺术是一种以风景为导向的光概念艺术, 而不只是强调风景。

created passing through为过去分词短语作后置定语。但这里似乎语法上有些问题。我核对了网上的阅读试题，原文也是如此。估计原文本身在这里遗漏了某个词。我就不做解释了。勉强解释不如不解释。对于阅读题不必过于细扣细节，知道是定语即可。