What does "play this ability only if you could play a sorcery" mean?
There are currently no cards with either the first or the second text.
From the Comprehensive Rules:
The Oracle card reference contains the official wording for all Magic cards.

108.1. Use the Oracle™ card reference when determining a card’s wording. A card’s Oracle text can be found using the Gatherer card database at Gatherer.Wizards.com.

Oracle has an advanced search tab with powerful filter options that allows you to search for cards with an exact wording. The wordings of cards get updates (errata) all the time, for example for cases like yours where a wording might be ambiguous, or when comprehensive rules changes affect a card's wording.
Additionally, many cards have rulings at the bottom of their Oracle page that answer specific questions that might arise from that card. Those rulings are not official rules, but they are derived by official judges by application of the rules.
In your case, the currently correct wording is &quot;Activate this ability only any time you could cast a sorcery.&quot;.
The term &quot;play&quot; is still used, but no longer for casting spells or activating abilities:

Play

To play a land is to put a land onto the battlefield as a special action. See rule 115, “Special Actions,” and rule 305, “Lands.”

To play a card is to play that card as a land or cast that card as a spell, whichever is appropriate. See rule 601, “Casting Spells.”

(Obsolete) Casting a spell used to be known as playing a spell. Cards with that text have received errata in the Oracle card reference. See Cast.

(Obsolete) Activating an activated ability used to be known as playing an activated ability. Cards with that text have received errata in the Oracle card reference. See Activate.

(Obsolete) The battlefield used to be known as the in-play zone. Cards that were printed with text that contains the phrases “in play,” “from play,” “into play,” or the like are referring to the battlefield and have received errata in the Oracle card reference. See Battlefield.