Timing of ADS-B messages
Physicus
Yes, you are correct and I was wrong. ADS-B messages do not have to contain timestamps and mostly do not. However, it is possible to include them along with any other extra pieces of information in optional parts of broadcasts.
The timestamps are added by the (very accurately timed) receiving stations for FR24 usually.
However, for the purposes of the last minute of the (recorded) broadcasts from QR8051, there are no repeat positions (except for the last two which we are ignoring). Thus it is safe to assume that there is just one ground station responsible for the timestamps.
Given the speed of light (meaning that time received is less than one millisecond from time broadcast), we can use the timestamps given as being perfectly accurate, even down to the millisecond. That is as far as calculating positions is concerned.