DD's stamina has increased for sure since we first started having "school time" in the evenings. We used to do it during DS's morning nap but when he dropped his nap we had to adjust, and she was initially a bit crankier in the evenings but she grew accostomed to it. Now she asks for school because it's fun and she gets to stay up a bit later.
But yes, her ability to focus has naturally increased over time for SURE, no doubt. A skill, like anything else, it just naturally developed.