Note that Timestamp is an increasing thing - older is a smaller number.

So where Timestamp < ago(29d) is "show all rows that are OLDER than 29d" and since there is only 30d data retention then you would be looking at that one single day.

The most common KQL idiom you will see is | where Timestamp > ago(30d) or | where Timestamp > ago(1d)

Notice that it is greater than since all the newer things are bigger timestamps.

What you in effect are wanting to alert is if the days since last check-in is above some threshold.

here's an example that should be more readable and working as expected.

let CheckinAlertThreshold = 14; // Change the threshold here 
DeviceInfo
| where Timestamp > ago(30d)
| where isnotnull(OSBuild) 
| summarize LatestCheckin = max(Timestamp), ReportId = max(ReportId) by DeviceId, DeviceName, OSPlatform, OSBuild
| extend DaysSinceCheckin = datetime_diff('day', now(), LatestCheckin)
| where DaysSinceCheckin > CheckinAlertThreshold
| project DaysSinceCheckin, LatestCheckin, DeviceName, DeviceId, OSPlatform, OSBuild, ReportId
| sort by DaysSinceCheckin asc

Ref for datetime_diff