Felonius said:
The argument used against this paragraph is that it talks about "filling" spell slots, i.e. preparing? Spontaneus casters do not fill spell slots, they just have this level spells that many times per day.
Who says that "fill a spell slot" means preparing?
I say it means put a spell in there for use as per the general definition of the word "fill". This is done instantaneously by a Sorcerer.
You can interpret fill to mean prepare and only prepare, but the book does not state that. If it meant prepare there, it should have stated prepare. Prepare has a specific meaning in the game and the word prepare is not used in this sentence.
You are also missing other quotes from the Spell Slot section that are STILL applicable to Sorcerers such as:
"A spellcaster who lacks a high enough ability score to cast spells that would otherwise be his or her due still gets the slots but must fill them with spells of lower level."
If they had meant Wizards only here, they would have stated Int instead of "ability score".
And it is nonsensical that Sorcerers and Bards lose their higher level slots for not having a high enough ability score whereas all OTHER spell casters (i.e. clerics, druids, paladins, rangers, and wizards) do not.
Felonius said:
Plus to make things even more unclear, it says this under Sorcerer class description:
"Unlike a wizard or a cleric, a sorcerer need not prepare his spells in advance. He can cast any spell he knows at any time, assuming he has not yet used up his spells per day for that spell level."
(emphasis mine)
This is the only rule that supports your interpretation. However, this can be interpreted two ways based on the rest of the rules:
1) Your way. That spell level slot and that spell level slot only can be used for spells of that spell level and that spell level only.
2) My way. Any spell level which is applicable. So, if a Sorcerer has a second level spell slot and the rules of spell slots indicate he can put a first level spell in that slot, then that IS a legitimate spell for "that spell level".
Or putting specifics into this sentence:
"He can cast any spell he knows [of zero, first, or second] at any time, assuming he has not yet used up his spells per day for that spell level [second]."
My interpretation is just as valid as yours with the English language.
Another note on this sentence: "a sorcerer need not prepare his spells in advance". This means he can if he wants to, so your "fill" argument is null and void.
If the authors had meant to change such a significant aspect of Sorcerers (and Bards), they would have explicitly put in an unambiguous sentence of:
"A bard or sorcerer cannot put a lower level spell into a higher level spell slot."
They did not do this, hence, I suspect you are making a mountain out of a mole hill here.
The problem here is that they did put in an ambiguous sentence in the Sorcerers section (the Bard's section states "the spell level" instead of "that spell level" btw).
It happens.