This has a major factor people like to forget; at no point do we know when the Guidances were recorded. They could've been recorded heavily at once, or slowly throughout it. DLC was still not outright planned from the start, even if Plant might've been. This means that Sakurai would never record a generic line for DLC unless he was sure it would exist. The only way it could've been done all at once is if it was very late into development, which is the only time we have information on Plant, a few months before the game's release.
Besides that, it was confirmed that every base game character was meant to be in there. There's also an old interview about Sakurai noting in 2017 is when Nintendo greenlit the DLC, but I can't re-find the source(I've shown Golden the link before, but yeah, it's a weirdly hard one to find). ...Which is far more likely when Plant was planned out(as the bonus DLC).
Regardless, it's still been a consistent thing that Sakurai doesn't start with DLC on the mind. But later on during development when it's greenlit. However, even Mewtwo wasn't done from the start, but later on, as confirmed in his own wording;
"
Mewtwo:
Sakurai: Among the veteran fighters who didn’t appear in
Smash for 3DS/Wii U, Mewtwo was particularly popular, and it was clear many people wanted him back. Unfortunately, it wasn’t possible to include him in the retail release. Thus, we decided that, if we were to release DLC, Mewtwo would be an effective veteran choice, so we began with him."
Sakurai discusses all seven of the DLC characters in Smash for Wii U/3DS.
sourcegaming.info
Notice how he says "if"?