It would appear that Microsoft is testing build 10586.164 for Windows 10 Mobile, a report from WinBeta states. The full build string is 10586.164.th2_release.160229-1806.
The new build is a cumulative update, meaning it likely focuses on fixes instead of features.
Updates are nothing new. According to Microsoft, Windows 10 is never done, so even after a major update is released, there will always be cumulative updates. Even build 10240 for PCs still receives cumulative updates.
This also doesn't mean that the rollout to older devices has again been delayed. There's no reason to believe that this build must roll out to Insiders before the Production rollout to older devices.
According to the report, Microsoft has been testing the update in its Windows Servicing ring since last week and it has passed on to the Microsoft-wide ring, which means that the next stop is the Release Preview ring.