I have one that I got for nothing (wouldn't have paid the asking price up my way - $80) and it does work fairly decently on plywood up to 3/8".
To anyone unfamiliar with the exact product it's just a metal (jig) that you attach to a circular saw to keep the blade at the equivalent of an 8:1 scraf angle (so if you try to scarf cut a 1/2" or thicker panel, the blade on a standard circular saw is not large enough to cut through the full 4" of angle - you can use the scarfer anyhow and finish cutting by hand with a router or plane, but the process is almost more akward than simply using something else in the first place).
You still need to clamp some kind of straight edge to the panel you are cutting to guide the scarfer.
You could easily make one out of a bit of scrap wood which would work every bit as well, and there are lot's of plans out there if you do a web search.
It is kinda heavy and as noted, probably not worth the cost if you are simply scarfing a couple of panels.
But it does work well on 1/4" and 3/8" panels and if you were having a hard time trying to figure how to cut a scarf correctly (some examples of how to jig up a plane are kinda confusing), it does take all the guess work out of the process.
Yes, Plywood is "real" wood
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it