The bands are said to be able to plug into the diagnostic port on the Apple Watch, making them compatible with the current-generation device. Letem Svetem Applem suggests that all of the health monitoring functions above could not be built into the Apple Watch itself, a supposition that may not be accurate.
When the diagnostic port was first discovered ahead of the Apple Watch's release, TechCrunch speculated it could be used for smart band accessories with additional sensors or external batteries, but there has thus far been no solid indication that Apple is working on such technology at this point in time.
It continues to be unclear what exactly the diagnostic port is capable of, but based on Apple Watch setups in retail stores and information discovered by third-party band makers, it looks like it's able to be used to charge the Apple Watch and relay data.
There are a number of reasons that Apple might be hesitant to embed multiple sensors in bands, including price concerns and durability issues. Apple would also need to consolidate bands or embed sensors into every band type, something that could be difficult with bands made of widely varied materials.
Letem Svetem Applem has accurately shared information gathered from resellers and other third-party sources in the past, but it does not have a known track record for sharing highly sensitive rumors that could only be sourced from a well-positioned insider within Apple. Because the site does not have a reliable track record, this rumor should be viewed with some skepticism for the time being.