Concern about key incompatibility between the base game (ARR) and Steam.

[Aishikai]

Summary of the lodestone post:

• It doesn't make any mention of product keys in the lodestone post. Exclusively licenses
  
• To verify you have a license, it asks you to visit a steam page that lists the games registered to your steam account. You can not view CD keys for any game from this section
  
• The very last paragraph tells you to contact support if the steam account you linked no longer has a FFXIV license. Further implying that the check is Steam license based, rather than product key related.

Furthermore:

"To link accounts, the Steam account must possess a FINAL FANTASY XIV license that matches that found on the target Square Enix account/FINAL FANTASY XIV service account (e.g., the Free Trial version or the full game version)"

• Once again, mentions a license, and even while mentioning it must match the one on the Square Enix account, it claims to ask for A license rather than THE license. This implies that the requirement is that the Steam account you're attemtpting to link must have either a XIV trial license, obtained by downloading the trial on Steam, or a full XIV license, obtained by purchasing the game on Steam

All of this makes me believe it will check for neither product keys or expansion licenses.
Only that the Steam account you want to link has a valid Final Fantasy XIV Online base game or free trial license registered to it.

However,
Square Enix should address this vaguely worded announcement and clarify with a follow-up post-haste for the peace of mind of its paying customers, including myself