In the Ranger Class Description Warden Spells are defined as a type of focus spell.
Certain feats grant you warden spells, which are a type of focus spells.
And the rules on Focus Spells define this for non-spellcasters:
You gain the ability to Cast a Spell and use any spellcasting actions necessary to cast your focus spells (see below). However, you don’t qualify for feats and other rules that require you to be a spellcaster.
The prerequisites on archetypes are quite precise in what you need to fulfill them. While some call for a specific class (Runelord) others call for the spellcasting class feature (Hellknight Signifier). There are also archetypes that require spell slots (Geomancer) or a spell repertoire (Wellspring Mage).
And then there is those that require the ability to cast spells from a defined tradition (Soulforger) or to cast Focus Spells (Magic Warrior).
Shadowcaster doesn’t specify any of this. It just requires you to be able to cast a spell. Even a Innate Spell gained from a ancestry would fulfill this requirement, as they allow you to cast spells.
You can cast your innate spells even if you aren’t a member of a spellcasting class.