I'd think there were two ways to produce this.

The first would be to not disclose the dose. If you don't know the dose, you might just assume it is a low dose and you aren't getting effects. This is especially easy to pull off since a lot of folks know about microdosing - which usually has the goal of not-too-much-hallucination.

Second, you can get folks new to LSD in combination with the above.

I'd think that the issue with giving another drug is that we wouldn't know how that drug is going to affect the outcome. You still need the baseline of sober folks to compare to, and given them a fake dose means that the ones that will have the placebo effect, will.