If I'm Sorc 5 casting fireballs, how does War2 improve my fireballs, or other 3rd level spell? Is this a reference to overstocking sorcery points through short rest spamming?
Since you can never have more sorc pts than the list, the res no overstock of those. It can be short rest reload of spent ones thru burning pact slots **if** the GM allows pact slots to feed flexible casting.
But even then the math gets iffy and situational
Sorc 6 has 6 sorc pts and 4 first, 3 second, 3 third level slots.
Sorc 3/bard 3 loses 3 sorc pts but get the same 4 first, 3 second, 3 third level slots and trades off sorc stuff for bard stuff. Lost the ability to cast actual third level spells but can cast lowers as up-slotted to third.
But sorc 3 war 3 has only 3 sorc pts but for slots only 4 first and 2 second plus 2 short rest seconds. Lost the ability to cast actual third and the ability to up-tick lowers to third.
So there are some hard set in stone spell caster hits - max spell level and max up-tick if you go warlock.
But if you look at the flex cast rest stuff...
Warlock gets 2 short rest second level slots but gives up three third level slots and one second level slots each long rest.
Just as flex-cast or total spells -
If you get two short rests, your total spells equalizes at four each but the sorc or sorc bard has higher slots mostly. The flex cast is 8 sp if two short rests vs 11 sp for the thirdsand second.
At three short rests between long rests you catch up with six total castings vs four and possibly flexing 12 so vs 111 (vs 14 for pure sorc if you count the 3 sp edge to start.)
As far as "spellcasting core" goes you gotta be pretty sure of three or more short rests per long rest for this to measure up.
Maybe one sees the warlock gains from invoc and pact/patron as superior to bard gains - but I dont. Too many gains.
Outside of white rooms, you gotta be looking at triple shorts or more regularly and aiming to spam a lot of lesser spells to get the warlock mc over the bard mc from sorc, imo.
Me, I take the guaranteed higher end gains from the sorc-bard over the maybe shorts in threes or more on the sor-lock for actual campaign play where the circumstances will vary.