Notes & Combos
I never had so much fun playing a deck more than this. You already gain so much advantage from activating the Runick spells and drawing from fountain, and now you also get to search for extra interruptions when activating them in the same column as one of your Muskets.
About Dancing Needle, I can't imagine the anger Runick players feel when chaining it to their field spell and banishing its targets. Using it once is enough to set them back so far in cards advantage that you've already won the duel if you resolve it.
I'm running Ultimate Slayer because it's one of my favorite cards in TCG and I did not know it was coming in the selection pack. Resolving it against a Despia board must be the most satisfying thing ever. Removing Mirrorjade without triggering it and also popping something with N'tss.
Top Secret Packs used
I can't imagine he is; Runick going second is always a pain, and considering the Musketeer's gimmick of activating their spells and traps from the hand, they almost always rather go first to even get just 1 monster on the field so they can deal with the opponent's as they attempt to develop it, instead of breaking their board after it's been developed.
DRNM and Ultimate Slayer are probably for when the coin flip is lost. They both have good synergy with Musketeers as the archetype tends to grind the opponent out instead of winning by damage (which is partly why they're so good with Runick). Opponent's monsters not being able to respond to both is also better than their backrow not being able to respond, since the Musketeers have a searchable speed 3 backrow negate.
However, I can't speak for the author. All I can say is I play a list very similar to this sometimes. I still prefer raw Musketeers myself, but I get how it's supposed to be played.
This looks like an interesting build, might combine it with another Runick Musketeer build I saw on youtube & see how that goes