Reddit accounts were associated because their professional name was plastered on products they each advertised, whether it was books/comics/memes/nfts/tshirts, it was all linked to the same personal brand(s). They also all shared mod duties on the same subs, and the way they wrote comments/conversed was very idiosyncratic, they had catch-phrases and quotes that seemed to be of their own making. The subs they spammed and account length were also indicators, because they almost always all crossposted to whatever subs would allow their content, from a sub only they managed, in order to drive content to their personal subs. Another thing is mod teams (I was occasionally in contact with over this spammer) would report the user as ban evasion and reddit admins would be able to associate the account, which is how most of their accounts get banned. We realized they always tagged their own accounts as "Quality Poster" on their subs too, a tag that was reserved for only their accounts and a few others, and it was pretty easy to tell the difference.

For the self-publishing side and professional pseudonyms, he would use the same publishing company between his different pseudonyms, which is something that book sites often encourage you to search based on. On lets say Book Depository for instance the publisher of a book is a hyperlink, so you can "find other books by...", so it was like different versions of his name with different sort of professional personas. LLCs like a book publisher are all publicly filed entities through the business registrar in their respective states, so you can find out who filed the taxes, which consultant helped create the entity, just by searching the LLCs name on the government website. (I actually reported someone to the FBI once in relation to an act of violence because they had bragged about their shitty personal business on their online profiles, which they stupidly registered with their actual name to their home address, instead of properly with a PO box through a registrar.)

So really all it took was clicking on his own link to his own book, clicking the publisher name, and seeing all the books listed. In other words, doing the very thing he wants users to do on the site. However anyone who did this and mentioned it on reddit would be reported for stalking/harassing him, especially if they shared some of his content he didn't think fit with his current persona. Ie a quote from the rape-pill book, or a comic he wrote making fun of people who protest police violence. It was all about crafting an image that redditors would approve of so he could sell his personal brand on the site and avoid paying for ad services.

I don’t know why Reddit would deal in bots when it has more powerful tools at hand.

Cheap, easy, impossible for outsiders to quantify, approaching IPO and cash to be made, keeping ad revenue the same without revealing how many actual users have fled the site, etc.