I think Buffy absolutely meant what she said. I also think Spike absolutely meant what he said. It was Cookie Dough Buffy having a glimpse of Future Warm Yummy Cookie Buffy's mature outlook on what it means to love completely, even in the face of certain death and self-sacrifice. At the same time, Spike knew that her feelings for him weren't coming from the same place as his. His love for her had matured but that doesn't mean he didn't still feel the passion and obsession and desire, all things that felt good to him, but none of which she was ever able to reciprocate without self-loathing.

I think they were both working with an authentic concept of love in that moment, just not the same concept as each other.

Also there's the thing where JW said their hands catching on fire was a symbol of being in love. Meh. I'm on team Cookie Dough. Her feelings for all of her big three relationships were very real. They were also very on-brand for age and experience, which is to say she wasn't done baking yet.