[Legion] Humanity didn't encounter Xenos until around M19

Context: Our old friend John Grammaticus is taking part in an investigation of Xenos ruins on a noncompliant human world, and one of the staff officers brings up something really interesting

Nurthene have no orbital or interplanetary technology, nor have ever possessed such means. However, the area known as Mon Lo Harbour, though flooded and used for maritime shipping, was originally constructed as a setting down point for starships.’ Uxor Rukhsana blinked. ‘For starships?’ she echoed. He was taking a slight risk in sharing this information, but John Grammaticus’s mind was finely trained to sort and appraise data. He knew exactly what he could give up and what he couldn’t. He believed it mattered very little if the Imperials found out that Mon Lo had once been an extraplanetary set-down. It was a halting site, in fact. The Cabal used to visit here, long ago. That’s why they knew about the Nurthene culture. ‘For starships, uxor.’ ‘Are you sure?’ Uxor Rukhsana asked.

‘Absolutely,’ Grammaticus replied. ‘I have excellent sources.’ ‘And when you say “originally”, Konig, what does originally mean?’ ‘It means something between eight and twelve thousand years ago, enough time for sea-levels to change, for flood plains to rise, and for a massive, stone-cut extraplanetary harbour to fill with water and become a harbour of a more traditional nature.’ It was eleven thousand, eight hundred and twenty-six years, in fact, and the construction work had taken eighteen months. Grammaticus felt it wise to fudge the precision of his knowledge. The aides started speaking all at once. ‘That would place construction during the Second Age of Technology,’ said one. ‘Around the time of the First Contact Event, and the first Alien Wars,’ said another.

I thought this was a pretty interesting tidbit of info I completely looked over on my first reading. Seems to track with how enormous the galaxy is and how humanity even in 40k is a comparatively tiny part of it, spread out along stable warp routes. Thousands of years of expansion before the first intelligent Alien contact seems fairly plausible. This would also place the contact millennia before the Navigators and DAOT itself, most likely when sleeper ships and other non-warp based travel methods were still in use. What do you guys think?