This is a book by Charles Oman, an Oxford scholar, which was written in 1893, first published in 1898 by Rivingtons. Charles Oman is introduced as M.A, F.S.A, Fellow of All Souls College and Lecturer at New College, Oxford. The language is fine and the book reads as well as if it were written these days. The narrative feels truly modern except for a few archaisms, for example: using the term 'host' rather than 'army' or 'ere' instead of 'before' and a number of some other.
The title of the book is "The Dark Ages. 476-918" and it has been made into an e-book in 2017. So this mistake in it might be due to the transcription of the original text into the electronic version perhaps.
scrapes some paint from a painting (icon?) he likes and mixes it with baptismal water
Yes, this makes sense to me as it goes well with rest of this passage on superstitions of the time.