I think the idea here is to reduce ad spend rather than to discriminate, but it has the side effect of the latter.

You can be perfectly willing to hire a 55 year old female construction worker, but you’ll probably save money by only advertising construction jobs to men aged 20-50.

It depends. If the goal was to exclude women or old people no matter how qualified they are then you're right, they could just be excluded at a later stage. But if goal was to save money or reach more relevant people based on the assumption that there are fewer old people and women with necessary qualifications then they actually might hire them once they've applied.

Now I think about it, even in the first case it could make a difference. Rejecting an application from a real individual based on prejudice might be harder to do, due to a stronger feeling of wrongness, than just unchecking a box labelled "female".

As someone who is not really young anymore, I would prefer not being shown those ads. They are not going to hire me anyway when I show up and they see me so I am just going to waste my time... and even if they were somehow forced to hire me, I don't want to work with people who do not want work with me. Some problems cannot be illegalized away.

While we're at it: why bother with any sort of anti-discrimination effort? Haters' gonna hate after all. /s