So, let me guess. You're new to this debate and you are only familiar with one side?
Let me recap this idea for you. This argument for the sphere came up in 2015 when flat earth started and was easily countered that same year. I'm surprised this guy isn't aware of that, but then again, when people are convinced that they are right they don't listen to the other side.
So this is a parallel line test. If the sun were millions of miles away, it would indeed prove an arc in the shadows. The entire premise requires the sun to be so far away that the rays come in parallel.
However, when the sun is right here in our own atmosphere, just like we see when we look up, that close proximity does not allow for parallel lines. A close sun would cause different shadow lengths whether it maintained the same altitude or had its own arced path.
There is nothing wrong with researching a counter argument yourself before assuming you understand an issue.