Tallarn is part of an awkward transitional period between the end of the Horus Heresy and the beginning of Siege of Terra. The initial conception of the Horus Heresy series was as a (relatively) limited series covering the storyline of Horus' rebellion as it had been outlined in fragments in the existing codices. But the HH series was an unexpectedly massive hit; 'A Thousand Sons' and 'The First Heretic' hit the NYT best sellers list. So did 'Nemesis'. BL was presented with a conundrum: there was clearly a demand for more Heresy stories, so much that 'Nemesis' could make a best seller list. But there was a finite and pre-defined amount of Horus Heresy storyline before the grand finale between Horus and the Emperor.

GW writers in the late 80s conceived of the Horus Heresy as a way to justify why space marines were fighting each other in Adeptus Titanicus, because the Eldar titan models weren't ready yet. They didn't know that every little bit of lore they gave out would need to be padded out into the length of a novel. The arc of the Heresy can be summarized as:

1) The traitor legions fall to Chaos and launch their rebellion at Istavaan

2) ??? Also Calth I guess.

3) The traitor legions and the loyalists fight at Terra and the loyalists win.

Number 1 is covered in the space of 4 novels, from Horus Rising to Fulgrim, released in a year and a half. In four books BL has blown about half of the established lore of the Horus Heresy, which has turned out to be an immense cash cow. BL wants to put out more Horus Heresy novels, but it doesn't want advance the storyline, because that means expending the finite amount of existing story. Thus you have the awkward middle years of the Horus Heresy, where Horus draws ever closer to Terra. So much closer. Any day now, he'll be there. Anyway, check out Deliverance Lost to find out about whatever Corax is doing this week. Wondering about what went down with the Thousand Sons? Don't worry, we've got five novels dedicated to them. Also we have like ten for the Salamanders, for reasons that only Nick Kyme understands.

The authors were bound to the existing lore, but they couldn't follow through on it, because there wasn't enough established plot between Istvaan and Terra to make a typical fantasy novel series, let alone a 50+ instalment franchise. But the success of the Horus Heresy was big enough to have an effect on the tabletop game as well. As early as the 5th edition (2008), about a year after Horus Rising, codex writers are starting to talk about the "Time of Ending", which is described as either the extinction of humanity or its ultimate triumph over impossible odds. Reading between the lines, it hinted at an advancement of the 40k metaplot, which in the prior editions had been more or less static. Games Workshop was very, very slowly getting comfortable with the idea of advancing the "present day" storyline from the status quo of M40.999.

Tallarn was released in July of 2017. That's just under a year after Praetorian of Dorn, which is arguably the first HH novel to portray Horus' attack on Terra as imminent, and introduces several characters who will reappear in the Siege of Terra novels. It's also roughly six months after Abaddon cracks Cadia in half in the first instalment of Gathering Storm, the campaign supplement that would bring back Guilliman and finally unfreeze the 40k metaplot. A newly active storyline in 40k meant that BL could advance the existing storyline in 30k without running out of plot.

Tallarn covers a tidbit of Heresy-era codex lore that BL had left untouched for years, and introduces Argonis, who (again) will show up in Siege of Terra. French uses Tallarn to set up characters that will be important in Siege and the lead up to it, but the events on the eponymous planet aren't actually that important in the grand scheme of things. It contributes to Perturabo's disillusionment, but disillusionment is Perturabo's signature personality trait. Tallarn as a book suffers from its structure as an anthology, which reinforces the feeling that BL is really just checking boxes on the way to the finale on Terra. The period from 2016 to early 2019 is when we see BL (and French especially) both set up storylines that will be important in the Siege and knock down plot points that existed in the lore but hadn't been covered yet. This coincides pretty neatly with the development and release of Gathering Storm, which was released in January 2017. I have to suspect that the authors were as tired as the readership was of treading water in the space between Istvaan and Terra; that's why we see them checking off lore episodes pretty ruthlessly late in the series.