Why do we care about which values are reachable by slider? If you needed to set a precise value, you wouldn’t go about it by carefully moving the slider with pixel-perfect accuracy…
It’s probably not about trying to get to an exact pre-known number, I suspect it’s more about resolution whilst sliding. Ie “this feels too opaque, I’ll drag it a little lower” and “this feels too transparent, I’ll drag it a little higher”.
Naive direct sRGB sample blending (what most image editing tools do) isn’t linear, some areas of the opacity slider have more effect than others. If you’re blending things into already gently changing areas of images (eg skies) then having resolution when adjusting opacity (eg of a cloud that you’re blending in to the existing clouds on the sky) is important to get things to match.
That’s why I love that GIMP and Photoshop provide an input box for entering numerical values.
I wonder why the author doesn’t mention GIMP anywhere.