Yes, I can see why this seems surprising. Ajahn Thanissaro translates pāduka as “non-leather footwear”. So anyone who follows his interpretation would be able to wear leather shoes. However, I have not been able to find any justification for his rendering. It seems to me that the two types of standard footwear mentioned in the vinaya, upahāna and pāduka, are best described as sandals and shoes respectively.
But I don’t think we should be too strict about this rule. The exact nature of upahāna and pāduka is bit fuzzy, and it would be a mistake to tie them too closely to sandals and shoes. So as with so many other minor rules we are probably better off asking what the Buddha’s broader intention was in laying down such a rule. From the context, the answer seems to be that pāduka were considered luxurious. Applying this to the modern context, I would say any footwear that is neither luxurious nor indulgent (this being the other standard criticism of monastics) is acceptable.
Also, the vinaya trends to be very flexible when it comes to illness or even discomfort. So for instance, if you use boots while working, I would say there is no problem. Likewise, if you use shoes or boots in a cold climate, then again this comes under the standard exemption for illness found elsewhere in the vinaya. True, it is not specifically mentioned for this rule, but considering its minor nature, I think it is fair to apply such standard exemptions.
So there you are. That’s my take on it. I tend to be quite flexible with the minor rules. There are some, however, who would disagree with this approach.
Anyway, congratulations on making good progress with the Camma-khandhaka! Please feel free to take your time. I would like everyone who takes part in this to enjoy their work.