OK - he said 'PBA' and I can forgive that. He's nervous. But then he goes on to talk about his little 4 micron silt filter as a 'purifier' when it's far from even being a microfilter. Truth be told, this prefilter likely does help address the issues with UV purification efficacy (or lack thereof) in silty water so that's good. But let's not confuse this with something that can be used on its own to provide any sort of reliable protection from waterborne illnesses. It's just a pre-filter.

Yes, you're right that it's definitely a pre-filter, meant to be used before a SteriPen. In fact that's what SteriPen says in their product description:

The SteriPEN® FitsAll Filter
The SteriPEN® FitsAll Filter removes particulates from water with the use of a durable 4 micron screen. The FitsAll Filter’s unique design offers the flexibility to work with a wide range of bottles. Water can be poured through the FitsAll Filter or users can immerse the FitsAll Filter and bottle into the water source. The FitsAll Filter also serves as an adapter on wide-mouth water bottles. After filtering the water, users remove the filter cartridge, activate either the SteriPEN® JourneyLCD or SteriPEN® Classic, and then insert the water purifier to form a tight seal and flip the bottle to purify. The bottle is inverted to ensure that the water contacts the water sensors and triggers the UV light. Enjoy safe drinking water!
MSRP $14.95

In testing the SteriPEN for my water treatment series, I found that their pre-filter, which has the same filter element as the FitsAll, does let a lot of pond-water scum through. It is, after all, a 4 micron filter, not 1 micron or smaller as are most pump filters. Since it is gravity-feed (you pour the water through the filter, or funnel in the case of the FitsAll), it can be pretty slow. It does clean mud-type sediment fairly well, and seems to do a fair job on some glacial runoff (glacial melt is often cloudy to the point of being translucent, and it came out fairly clear after running through the filter). That seems to be good enough for the UV treatment, if you stir pretty thoroughly. In both the glacial and muddy cases, I gave two full exposures just to feel safe.

Can this be used as an adapter for my SteriPen Adventurer for use with a wide mouth nalgene?

WELL... I use a little plastic funnel (probably toxic plastic) and a #2 coffee filter to keep bugs and small children out of my bike bottle when filling it for use with a SteriPen. That's good enuf fer me.

Eric