Citing a blog post by the Brookings Institution is very funny to me. They were one of many think tanks in Washington that built the intellectual groundwork to justify the expansion of NATO in the first place.
There is no shortage of sources on the ambiguity of this alleged promise [1][2]. More critically, there is a total absence of sources from Putin. Because he doesn't have any. Because when nations agree to things, they do so by issuing public declarations and/or signing treaties.
[1] https://nsarchive.gwu.edu/briefing-book/russia-programs/2017...
[2] https://www.nytimes.com/2022/01/09/us/politics/russia-ukrain...
.. signed in 1997, which fixed the principle of strategic partnership, the recognition of the inviolability of existing borders, and respect for territorial integrity and mutual commitment not to use its territory to harm the security of each other.