Yes you are correct it is not an attempt to predict travel time under "current" traffic conditions. It is simply a measure of how quickly (slowly) traffic is moving along those roads at times when it tends to move its slowest. There is no specified time interval.
It's not like we say "rush hour tends to be at 5:00, so let's measure the speed of traffic at 5:00 every day for a month and say that's the time in traffic". That obviously wouldn't work, because rush hour times are a little different everywhere. Think of it more as if we said, "when traffic was its worst today, how slowly was it moving" and answered that question every day for a month. By coincidence, the time when that happened would tend to be rush hour, but that time could be different depending on the city, road, and direction of traffic. It's more sophisticated than that, because as I said we don't want to include extraordinary events such as accidents, but that gets the general idea. The point is, that time is intended to be the worst case driving time barring extraordinary events.
in the office.