Ah thanks for the link to the working paper, I had been looking for it. This jumps out from the footnote on page 7:

"As far as we know, this time is not available on any publicly available database. We initially obtained these times using real time “scrapes” of the SEC EDGAR site. We subsequently used a collection of these times made available by the Tier 1 subscriber. Given that this entity’s business model depends, at least in part, on obtaining and disseminating these filings in the most timely manner possible, they have strong incentives to collect accurate information about when filings become available on the SEC website."

Basically, clock skew is not accounted for at all and timings are potentially done from disparate and non-common observation points. It's quite possible that the delays they observe are strictly due to noise.