Once the SMM boils out it doesn't matter if the condensation drips back in.
In theory, that's correct from what I've read. If you boiled for 60-90 minutes uncovered and then put the lid on and continued to boil, there would be little/no SMM left and hence no DMS in your beer.
Now, if you have condensate with SMM dripping back in throughout the boil, you're only removing a percentage of it and it's going to take longer for it to be reduced/removed via boil-off. How long in practice probably depends on a lot of factors.
So what makes both sides of this argument seem a bit dubious to me is that there are assumptions being made regarding the behavior of SMM in its vapor state. Once boiling, the SMM and water vapor are separate - the SMM is no longer dissolved in the water because they are both in a gaseous state. When the water condenses on the lid it doesn't contain any SMM immediately. The SMM would then need to redissolve in the condensate, or condense out itself before redissolving. The rate at which that happens will be determined by things like boiling point, hydrophilicity, etc of SMM.
I'm not sure the research has been done already, but I would imagine you would need to test for varying factors (length of boil, boiloff rate, atmospheric humidity and temp, surface area and shape of lid, yada yada), then run the condensate through HPLC to determine the amount of SMM over time. Or you can just brew some beer and use your palate.