It's OK to get distracted temporary and/or even get "excited".
What is sad - very sad - is that right here at TB with all that "so called" KNOWLEDGE of Harmony, theory, etc..., a TB member could write a "strange" statement that "A Mixolydian must have three sharps," or D Dorian has one flat.
Any knowledge/understanding about MODES is absent, but...
It's getting sadder that two TB members (that I highly respect) "liked" it.
I think somebody just "presumed" that the melody is A Mixolydian.
Just by listening to that very simple melody, one could easily find the following notes:
A melodic phrase starts on C# and ENDS on A or a slightly developed phrase starts on F# and ENDS on D, plus there are E and B notes.
Let's take those "strong" notes A - C# D - F# and play as a CHORD.
P.S. That's my favorite inversion of Dma7.
N.B. I could not hear even ONE note G - a very important "flavor" note for that "A mixolydian".
P.S. This song - Franklin's Tower - has more relations to some Eastern European POLKA than to some American Blues, but I think you knew it already.