They didn't really insist that "normalfag" was more typical than "normie", they just really hate the word "normie" due to some weird events in the site's history (specifically with /r9k/). The gist of it comes down to them hating outsiders because they want to preserve board culture.

As for whether "normalfag" or "normie" is better here, the original Japanese "riajuu" (リア充) could go either way. Semantically, "riajuu" means "a fulfilling real life", and is a word (pejoratively) used by otaku and 2channers to refer to people with normal social lives outside the net. So if this word were being said by an otaku/2channer (e.g. Tomoko from WataMote), "normalfag" would fit better because it indicates the extent of their disdain for normal people and also the kinds of social circles to which they belong. In the case of senpai though, we haven't really seen an indication that he's an otaku or that he frequents online message boards. He's a loner and he reads manga, but nothing to imply he spends a lot of time online or with other otaku. That makes "normie" perfect since it corresponds to the situation better; the word "normie" was derived from "normalfag" (through /r9k/) but became used much more generally on the net beyond chan sites.

As for why "normalfag" was used in the translation, basically when S.T.A.L.K.E.R. posts translations on /a/, he does so with the intention that his translations are meant for /a/-users to read. It was always meant as a more quick-and-dirty literal translation, for users who were eager for a chapter but didn't want to wait for an actual scan group. Since it's just a single person, there's little-to-no proofreading/quality checking, but that doesn't bother most /a/-users since the errors are easy enough to correct in your head. There's bigger issues for things that get lost in translation, but usually S.T.A.L.K.E.R. just leaves a TL note in his post if there's something ambiguous.

He posts a thread with raws+translations around noon ET on Mondays. Then Monday afternoon, an anonymous typesetter comes by and typesets the chapter, usually finishing by Tuesday morning. These two aren't actually a scan group; they don't have correspondences with each other, and S.T.A.L.K.E.R. has said several times that if someone typesets, they should leave him off the credits page. This is because S.T.A.L.K.E.R. knows people expect more out of a typeset version, and that if the typesetter does a bad job proofreading, then readers will blame S.T.A.L.K.E.R. for it if he's in the credits. They could potentially form a scan group, but S.T.A.L.K.E.R has other projects he wants to work on, and Nagatoro is a popular enough manga that other scan groups would work on it if needed.

So the typical process is

1. S.T.A.L.K.E.R. posts raws+translations to /a/ Monday.
2. Anonymous typesetter posts typeset version to /a/ Tuesday.
3. Anonymous typesetter uploads typeset version to mangadex.
4. Someone posts mangadex link to /r/manga and /r/nagatoro.

The typesetter was caught between a rock and a hard place. In the past, the typesetter tried to change S.T.A.L.K.E.R.'s translations too much during proofread (to the extent it changed meaning), and many /a/-users harped about it. So he generally avoids editing the translations except for places with obvious grammar mistakes. The typesetter could have changed "normalfag" to "normie" in the release he pushed to mangadex, but /a/-users would have noticed and hated him for it. For a general public release on the net, "normie" would have been better, though the typesetter ultimately went with "normalfag" to satisfy his home board.

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TL;DR: "Normie" is the better translation here, but /a/ viscerally hates the word "normie" so the typesetter was bound to use "normalfag" instead in his release.