As someone who has implemented tables in a browser engine, the idea that flexbox layout just duplicates table layout is completely untrue. Tables are extremely complex, don't have a complete specification, and differ between browsers.
Just to give one example, the algorithm for computing widths of table columns involves creating four "candidate guesses" of all the column widths using different algorithms for each, selecting the two guesses that most closely bound the available width, and then linearly interpolating between them [1]. And that's the easy case, when there are no colspans: when you get colspans, the layout parameters are defined by a complex recursive series [2]. Flexbox has none of this complexity: it basically does the simple thing you'd expect.
[1]: http://dbaron.org/css/intrinsic/ section 4.3
[2]: ibid, section 4.1, "intermediate intrinsic minimum width for span N (N > 1)"
