I wish that were true but I don't think it is that simple. If you get newly hatched chicks who have never been outside a brooder you can introduce them whether they are vaccinated or not and not worry about the rest of your flock.
If your unvaccinated flock has Marek's and if the newly hatched chicks are unvaccinated then the new chicks might be in danger, but the unvaccinated flock are not at risk from the new chicks.
If you introduce an older chicken it is more complicated. If the older chicken has been exposed to Marek's they might infect the unvaccinated members of your flock. There is really no way to tell if the new chicken has Marek's apart from having their blood or mucus tested in a lab (and of course even that isn't 100%).
If they have a super severe strain of Marek's and are not vaccinated they will probably die while in quarantine so then you would know. And if they are very old and not sick they probably (not definitely) don't have Marek's.
Otherwise they might be vaccinated and living happily while infected. Or they may be unvaccinated and have Marek's and infect the other chickens.
Yes - I have read that the Marek's virus is so widespread that you can sort of assume that the chicken has had Marek's. I have never heard the 30 weeks timeframe for that.
Because of the nature of the disease however a living chicken who has been exposed to Marek's will likely be passing it around to its friends. It isn't like a cheap vaccine, the virus is in them and might cause them problems, or might be passed on to other chickens where it might cause problems.
The only thing you can know for sure, is that if they encountered a very virulent strain and are not vaccinated then they will die very soon.
Sorry to sound so wishy washy about this - but this is why there is so much controversy about the whole topic, it is genuinely tricky to know what to do.
Personally I am very cautious. I have my birds tested, I made
@bgmathteach test her birds before I took them, I test every chicken that dies and I introduce vaccinated chicks. My reading of the evidence is that for a small flock owner vaccination is by far the best way to go. Many have concluded the same as me, but some go a different route and accept there will be losses.