Yes, it was. It's still true, though, that the number of non-Core Rulebooks to hit 100k sales, ever, is tiny. Prior to 5e and since the fad in the 80s ended, that list may well have zero entries. If it's non-zero, it's very short.
And now, with 5e, WotC hope to his 100k sales with every book? Are you seriously saying that's not an ambitious target?
In terms of sales, not really, not compared to past targets. For a $50 book, 100k units is $5 million. They're only doing a few books a year, and they aren't all $50 nor nearly all selling at full price. So we're looking at an annual sales target in the ballpark of 10 million. That's not unreasonable or ambitious for a product line that regularly takes the lion share of it's market, now that that market is back up to around $25 million. Maybe it's a little ambitious to do so with so few books, but, if you look at it as what fans are willing to spend on their hobby, maybe it's not that unreasonable nor even ambitious to try to get 100k of the 800k or million or two folks who play the game to buy two or three books /a year/.
True. That's why I didn't include 1st Ed in my list. "Tomb of Horrors" is thought to have sold ~250k copies, "White Plume Mountain" ~175k. The biggest seller is "Keep on the Borderlands", at around a million copies...
So adventures did sell more than 100k, sometimes a lot more, back when there weren't six official settings being hawked simultaneously and splatbooks weren't a monthly occurrence.
While, admittedly, D&D hasn't returned to fad-years levels of popularity, it has returned to a publication model that's closer to what it was when modules were moving a lot of units. There are still differences - the modern modules are called 'adventures' and published in a hardbound format, they cost more but represent much longer campaigns - but, the slower pace of publication means that there's less official D&D material competing for the fanbase's gaming budget.
Maybe existing competition from Pathfinder or a hypotheticall 5e 3pp will step into that void and spoil the strategy, or maybe not.
For 5e to do that with every book would be quite something.
Not when it's only a few books a year. If it were a book a month, yes, each book selling 100k units would be amazing - and D&D would finally be pulling in close to the kind of revenue they pitched to Hasbro in 2007. That would be extreme ambitious, to the point of being totally unrealistic, but 5e isn't trying to nor expected to do that in order to be considered an unqualified success.