It was probably a cheap easy way to get a bike rack. Wood, only a couple bucks. Likely the rear supports were from a broken, possibly free, rack. Probably taken from the trash, could be old fender struts.
Almost no work, Drill two holes.

But, how do you ride with a top tube a little smaller than 4" ??? Sounds uncomfortable, but I have never tried it. Except for the owner of that bike, most likely no one in the world has ever tried it. Maybe going slow and short distances it's not so bad.

I hope it's pressure treated. Maybe a good part for drillium?