there are two separate things here.
"on or about" (s/on/in, in this case) is an expression[0] that indicates an approximation. hedging in case it actually happened in, say, 2018 or 2020.
the "at least" is what you'd think, of course. combining the two in this way feels awkward, but it's common usage [1].
[0] https://www.nolo.com/dictionary/on-or-about-term.html
[1] https://www.google.com/search?q=%22at+least+on+or+about%22
there are two separate things here.
"on or about" (s/on/in, in this case) is an expression[0] that indicates an approximation. hedging in case it actually happened in, say, 2018 or 2020.
the "at least" is what you'd think, of course. combining the two in this way feels awkward, but it's common usage [1].
[0] https://www.nolo.com/dictionary/on-or-about-term.html
[1] https://www.google.com/search?q=%22at+least+on+or+about%22