I think this was more his graphic design style for his books. As you say, the justification shows there's nothing impromptu about it. Here's how he wrote his letters, notes, drafts and so on: https://www.grough.co.uk/lib/img/editorial/wainwrightmanuscr...

I'm not splitting hairs on what 'handwriting' means. I'm fascinated by that era when books with complex text/visual interactions were far cheaper to create by hand. An example of an old magic book I have: https://imgur.com/a/8Bu9ML1 It's hard to beat Wainwright though.