Yes, the page is a photocopy from an original handwritten manuscript of Li Yiyu’s. I suspect that the larger characters and markings were added by someone else’s hand, but I’m just guessing. I’m not that good at distinguishing different people’s handwriting!
I’ve seen this kind of annotation marking before—the three vertical circles. In fact, I have a copy of the Hong Kong edition of Wu Gongzao’s book on Wu style taijiquan, which is particularly valuable because it has a color photo-reproduction of Wu Jianquan’s handwritten manuscript of the Yang Forty Chapters. There are a number of pages with this three-circle marking next to key terms, such as “dongjin.” Additionally, the three-circle mark sometimes appears at the top of the line (in the ‘margin’) where the term appears. The annotations are in red ink. That could be the case with the Li Yiyu page as well, but of course we can’t tell because it’s monochromatic.
These marks are, I think, analogous to underlining, as Jerry says, or highlighting. I work my books over pretty well—underlining key passages, scribbling notes or marks in the margins—so that when I review it at some latter time it is easier to locate what I keyed in on in my initial reading. That’s probably what’s going on here.
In any case, it is fascinating to look at this page with the knowledge that it was produced from Li Yiyu’s own hand. Gu Liuxin remarked that it is not known what eventually became of Mr. Yao’s copy of Li Yiyu’s full manuscript, or of the rest of his library. It is sad to contemplate the extent of what has been lost over the years due to political turmoil. So much the better, perhaps, to think about how much remains.