Not entirely true, but not entirely untrue either. In my opinion, lots of Jet tech was more stable, featureful, and performant than what Walmart had to offer. But in many cases, politics prevented them from taking full advantage of Jet's tech.
In my experience, Walmart tech was very resistant to letting Jet run services that the Walmart website had to rely on (not in all cases; pretty sure Walmart is using Jet's inventory system: good presentation on it here: https://www.youtube.com/watch?v=dSCzCaiWgLM). I think there was a feeling like we were encroaching on their territory. Think about it: Walmart buys Jet, Marc Lore is made head of Walmart eCommerce, layoffs happen on the Walmart side shortly thereafter. Not exactly something that will foster warm fuzzies.
Walmart also gets WAY more traffic than Jet, and I think there was a little bit of dismissiveness caused by that too. A sense of, "We are the big fish, we are not doing anything wrong, and we are just going to keep on doing what we're doing."
I'm sure there were other areas of close collaboration, but I didn't see too many of them. I haven't worked there for five or six months, so things may have changed, but that's how it was from my perspective.
I did love working there though, was a very transformative time for me as a technologist.
Are you an insider? Where have you heard that Walmart was unable to use any of the jet.com technology?