Oh my, they are not the first band who have used the same chordal structure twice. Or a hundred times like John Lee Hooker.
Of course they have a stronge grunge influence. In their production, the bar chording guitars, the in-your-faceness, even harshness of their music, some of the melodies, the use of acoustic guitars, even Chad's neo-Nirvana haircut. When I first heard Nickelbag 10 years ago, I thought: "Ah, grunge has gone power pop." It was the resaon why I did not like them because at that point I still had issues with grunge, Nirvana, Soundgarden, it was all bad harmonic combinations to me, key-alien chords, minor where major should be, etc. It took me the better part of twenty years to get used to that form of music and I still wince at some harmonic changes to this day, I'm not very adventurous in that way, it's also the reason why I prefer Deep Purple over Led Zep, the former have an almost rigid harmonic concept, very much in the European classical vein while Zep had more harmonic freedom (which, however, did not appeal to me, you could never really tell just by listening whether their songs were in a major or a minor key, with DP it takes you seconds to realize that Woman from Tokyo - though the move form the E major riff to the verse in G major and F major is not according to the harmony rule book either - is major while Smoke on the Water is minor, never mind how the G# major in the chorus is a bit adventurous too).
We have a song, I even wrote it, where the chorus is in D major / F# minor / C major / A major. It was my conscious attempt to do something outside of traditional harmony sequence. To a grunger that must sound innocent enough, but to this day when I change from the F# minor to the C major, which is actually the diminished fifth (which was once forbidden to play in European classical church music as the "note of the devil") I hold my breath and think to myself "I hope this is going to work out now ...". I'm always expecting this big voice from the sky going: "Uwe, that C major does not belong there and you know it, play either a B minor or a C# minor there for Chrissssakes, but C major is wrong, wrong wrong!"
When I played the same harmonic sequence D major / F# minor / C major / A major to my Britpop bandmates a few years back their immediate comment was: "sounds grungy there ...".
I've even gone as far as buying the new remastered version of Nirvana's Nevermind in an earnest attempt to understand the appeal of their music 20 years after the fact. I'm still learning ...
And to get back on the subjéct, Nickelbag are far more conventional in their harmonies than either Nirvana or Soundgarden, but they still have the odd grunge chord in there sometimes. They are not Journey "Don't stop Believin'" you know.