almost all the old tools I have seen, plane iron, chisels, plough plane irons, had a rounded convex bevel, and Paul Seller's knows how to make that work and fast.
I would be hesitant about drawing conclusions from looking at old tools. When was the tool sharpened? If you get a tool that hasn't been sharpened in 200 years, then you can be sure it was sharpened by someone who really knew what they were doing, but it is very unlikely you will find a tool in that virgin state. Much more likely is that the tool was sharpened some time in the last, say 50 years, by someone who did NOT know what they were doing. As Warren has said, the dark ages of hand tool woodworking lasted over a century.
If we look at old texts, like Moxon, Nicholson, Holtzappfel, etc., we don't see any evidence for the convex approach. We see suggestions for either hollow grinding, or a shallow primary bevel with a steeper secondary. I do recall seeing a post by Jeff Burks of a late 19th/early 20th c. text that warned against the convex approach.
I'm positive that the craftsman of old didn't have honing guides or really paid much attention to whether their plane blade was 25 or 33 or 35.666789 degrees. Those craftsman seemed to make great works of art. Just saying..
I am positive that they DID pay attention to the angles they were using, though you're right that they didn't have honing guides. Again, we can consult the historical woodworking texts and find that they were quite precise about degrees. Nicholson, for example, recommends 35° for a bench plane. English common pitch is 47.5°, so that leaves a clearance angle of 12.5°, which is within 1/2 a degree of the 12° clearance that Stanley and Veritas use on there bevel-up planes. Probably not a coincidence.
"For me, chairs and chairmaking are a means to an end. My real goal is to spend my days in a quiet, dustless shop doing hand work on an object that is beautiful, useful and fun to make." --Peter Galbert