My sense has been that they don't mention anything about avoiding spoilers to the guests beforehand. It always seems very much like any pre-discussion doesn't get into many details, beyond "watching the series in order, one episode per week, we take listener questions." Whenever they get to the individual segments, Garrett is always explaining what they are to the guests in a way that sounds like the first time it's being brought up.
I'm basing this on nothing, so I could be 1,000% wrong, but I always imagined that they just didn't want to make it sound too involved beforehand. "Hey, come on our podcast! Here's several rules you have to follow. First, you have to make sure not to spoil anything..." And of course we all know how hard it is for the cast to remember what happened when (reasonable), so they try not to make it sound restrictive and then the conversation goes where it goes.
I actually think it'll be good for the discussions that Robbie and Garrett got spoiled for the genetic enhancement. Several times so far on this pod, they've been having interesting chats about Bashir, that then hit a wall when Armin or Terry can't say anymore without giving it away.