It's up-to-date with regards to the fact that the survey is rolled out every month, but like all surveys, it's a sample of a population.
The latter, in this case, is the entire database of Steam user accounts; the sample size is harder to define, and for two reasons. Firstly, we have no idea how many sample points Valve uses to determine the statistics shown in the survey: it could be a thousand or ten thousand, or anything. Secondly, the survey requires a 'sample' to actively respond, so even if Valve attempts to garner ten thousand samples for the statistics (which would be more than enough, even for the size of the user database), there's no guarantee that every account selected will complete the survey.
But it's all we have. Despite numerous game and software developers/publishers sampling hardware and system specifications on a routine basis, the public never gets to see that data. The next best thing we have is the 3DMark database, but that has the same issues as Valve's. It's updated more frequently (it's a continuous process) but again, it requires active participants to download the software, run it, and all it to upload the result to the database.
Unfortunately UL doesn't offer enough temporal statistics, for my liking at least. For example, one can bring up a particular graphics card such as the RTX 3070 and you'll be shown a Popularity Rank graph:
NVIDIA GeForce RTX 3070 Graphics Card review with benchmark scores. See how it compares with other popular models.
benchmarks.ul.com
But what exactly does UL mean by Popularity Rank? It seems to be based on the % of that week's 3DMark entries coming from that particular GPU. One can also list of all of the GPUs recorded in the database and rank them by popularity:
See the best Graphics Cards ranked by performance.
benchmarks.ul.com
But again, that seems to be based on that month's % of entries. An RTX 2060 is down in 15th place, but there's absolutely going to be more of those installed in active PCs around the world than, say, RTX 3090s (which is 2nd).
So for all its flaws, the Steam Hardware Survey is the best measure of the distribution of CPUs, GPUs, system configurations, etc that we have.