The 125m figure is referred to as "lifetime active users" on many articles reporting it, there's no time limit on this figure and from what I've found it seems that this is just the number of steam accounts that have made a purchase. The 67m MAU's figure I linked in the post above is more up to date and makes more sense.
Steam had 67m MAU's as of August 2017.
I just double checked. You are entirely correct.
However... Going by the statistics, concurrent daily players has increased by 32% (Rounded down) since the last time the 67~ million figure was thrown out, so we could be looking at closer to 90~ million monthly active users now.
That still ignores the extra users on Origin, uPlay, Battle.net, Gog.com, Windows Store and so on.
But that traffic wouldn't be routed through the same place. Which is exactly the point I was making. All PS4 games and patches can only be downloaded from one place, PC gamers have dozens of options.