What they mean by "change the damage calculations of tanks"?

[Nektulos-Tuor]

Advanced Materia Melding specifically refers to slots beyond the base slots on items; i.e., the 2nd through 5th slots on accessories.

Even being able to slot one slot with strength is still extremely powerful.

http://xivdb.com/?item/5608/Strength-Materia-V

When better crafted Jewelry comes out, which will probably have 40-45 vitality for example, you can add another 15 strength to that item. Now that item is more powerful then all pieces of raid jewelry in the game, just like before.

We will have to see, because this was a translation.

[Alahra]

Even being able to slot one slot with strength is still extremely powerful.

I never said it wasn't. I'm only saying that, according to the information we have now, we'll still be able to meld one primary stat materia to each accessory. They could have mistranslated, but the slide from the Live Letter presentation also mentions that we can still affix primary stat materia to "unfilled slots," which is unclear but almost certainly means the base slots.