It's not uncommon to write a classical Chinese poem in a Japanese scroll, so "a real Chinese poem is used there" doesn't prove that it's a Chinese scroll. As for your scroll, as Majestic-san mentioned, 兰 is a simplified Chinese character mostly used in Mainland China or Singapore, and never used in Japanese. Similarly, 异 is used as an alternative of 異 there, but this usage is found only in modern simplified Chinese. 异 is also used in Japanese, but it can't mean 異.