Alpha blending is:
Cout = α * CFG+(1-α) * CBG
Now there are two alphas on the image: a full transparency and the object transparency. The first order of business is to calculate the full transparency. Because we know the color and structure of the checker, we get a system of equations out of two pixels that have the same color.
C = α * CFGdark+(1-α) * CBGdark
C = α * CFGDLight+(1-α) * CBGLight
In these 2 equations there are 2 unknowns, so first let's solve for alpha. Since C is same in both equations we get:
α * CFGdark+(1-α) * CBGdark = α * CFGDLight+(1-α) * CBGLight
And then:
α * CFGdark+ CBGdark - α* CBGdark = α * CFGDLight+CBGLight
-α * CBGLight
Collect by terms:
α * CFGdark - α* CBGdark - α * CFGDLight + α * CBGLight= CBGLight - CBGdark
break out alpha and divide
α = (CBGLight - CBGdark) / (CFGdark - CBGdark - CFGDLight + CBGLight)
Whew, now that we have calculated the alpha we can go use the first formula and solve it in terms of CBG:
Cout = α * CFG+(1-α) * CBG
collect by terms and divide:
CFG = -((1-α) * CBG - Cout)/ α
OK let's test this! Take the images:
bg.png and 0.5.png (click to zoom)
Where the second is a synthetic check with 50% opacity stroke on the background. We can use ImageMagick to do the calculation. Run the command
magick 05.png bg.png -fx "-((0.5)*u[1]-u[0])/0.5" out.png
you get out.png

Seems to work rather well. You can now do a difference alpha for an alpha mask.
Now the original image does not seem to be alpha blended. Maybe the mode is multiply or screen? Let us test those next.
Multiplication mode is:
Cout = CFG * CBG
Thus
CFG = Cout/CBG
Testing
Results in:

looks ok. But thats not it either. Maybe multiply with a blend?
Screen mode is:
Cout = 1 - (1 - CFG) * (1 - CBG)
So:
CFG = (CBG - Cout) / (1 - CBG)
But that wouldnt be recoverable.
OK so not easy. But theres still stuff to go through. Maybe they have multiple alphas? What if the multiplication or blend was done color correctly and not naively like photoshop software tend to do? (Need to do actual work also sorry)