When dealing with a perfect circle (in your case, a hypothetical lake), then the following applies:
Circle Circumference = C
Circle Diameter = d
Circle Radius = d/2 or r
Circle Area = A
Circumference (C) = pi * (d)
A = pi * (r)^2
Therefore, attempting to interpret the formula in your question, the following is offered:
For a perfectly circlular lake, L = C = pi * d
Finding L (or C) when area of the circle is given, A, follows:
A = pi * r^2, and r = square root (A / pi)
L = pi * d = pi * 2r = pi * 2* square root (A / pi)
L = 2 * square root (A * pi)
Now, getting back to the equation:
L - 2 * square root (A* pi) = 0 for a perfectly round lake.
Let's try your questions with the crater lake having L = 26 and A =21:
26 - 2 * square root (21 * pi) = 26 - 16.2 = 9.75 and not equal to zero, and not a perfect circlular lake.
For a perfect circle with circumference = 18.849, then area (from equations above) = 28.27
18.849 - 2 * square root (28.27 * pi) = 0 and therefore, a perfect circle.