Quite the reverse. That said, a lot of indie publishers did try to codify "fiction first" principles into the rules and play procedures rather than "that's the way that it's always been played."
I have told you before but our memories about our pasts tend to be highly deceptive. We often project false histories onto the past based upon the present, and the more time between the present and a given moment in the past, the greater the potential room for misremembering. It's no secret to scientific studies that our memories play tricks on us and cause us to misremember in a variety of different ways. This is pretty clear when some American politicians project a utopian like society on the 1950s.
Sandboxes were created in 1850 in Berlin's parks. (Yes, sand existed in enclosed spaces before that.) Sandboxes have served as a metaphor for creative interaction and play since even the late 1800s. So it is no surprise that "sandbox" existed as a term in TTRPG circles; however, in earlier publications its meaning was closer in meaning with "the campaign" or even other more generalized uses rather than its present specialized meaning describing a particular playstyle or setting.
Dragon #25, Tim Kask
Dragon #247, Page 123
Later in the issue
This latter, more restricted meaning supposedly came more directly from video games. According to designer Robert Conley:
Most of what I have found on the Internet seems to conform with the above point that our current sense of "sandbox game" came from video games, even if both "sandbox" was used and this style of play existed in TTRPGs prior to its coinage. (I also saw one TTRPG source use "story telling game" back in 1980 for what we would now clearly call a "sandbox game.")
The above also matches up with the development of the term "sandbox games" in video games:
"The Theory and History of Sandbox Gaming" -
This article points out that while games that we would retroactively consider as "sandbox games" existed prior to the coinage of the term, it was only with the advent of The Sims (2000) and Grand Theft Auto III (2001) that we see "sandbox game" coined to describe a style of video game. These are also the two games mentioned as redefining the genre on the Wikipedia article on
Sandbox Games. And unsurprisingly both of these games predate the Wilderlands of High Fantasy (2005) book by Necromancer Games.
Even if you search for "sandbox" on the ENWorld forums from about 2005 back, no one is really using "sandbox" with this more specialized meaning. It's much closer to what we find in the Dragon magazine snippets above, where it's used more akin to "playing in someone else's sandbox" (i.e., a game, a campaign, a setting, the table, etc.) rather than its more contemporaneous sense. In one such post, Eberron is described as WotC's "sandbox," with a meaning that is closer to what we would now probably refer to as a "kitchen sink setting."