This appears as scene S21 in the game's text files. To trigger this, you need to beat the game in the Story Mode after having purchased literally every single upgrade in the game. This isn't possible if you beat the story mode as a single credit clear as you can't get nearly enough energy tanks in one playthrough to do it, so what you have to do is beat the game, then replay your cleared save file and grind levels until you can buy all the remaining upgrades (Stage 5 Deja Vu seems to be the best level for getting energy tanks).

It's kind of a silly way to hide a bonus ending scene. I was hunting for this expecting it to have requirements you can trigger in a 1CC (no continues) clear. Ah well. Note that stages 2-2 and 2-6 also have slightly altered dialogue if you beat the game without ever upgrading or pausing the game in the middle of a level.

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