Does Guardian Angel prevent Lonely Poro from drawing?

Yes, Guardian Angel prevents Lonely Poro from drawing a card.

Reasoning

• Replacement Effect: According to RiftJudge FAQ #9367 and Rule 369.1, Guardian Angel acts as a replacement effect. When the unit would die, the effect replaces that "death" event with "kill this Guardian Angel instead," and the unit is healed and recalled to your base.
• Death Prevented: Because the unit is recalled to the base, it never actually "dies" or is sent to the trash.
• Deathknell Requirement: According to Rule 808.1.d.1, the Deathknell trigger condition is specifically the permanent being killed and sent to the trash. Since the "killed" event is replaced by a recall, the Deathknell condition is never met, and the ability does not trigger.

This interaction is consistent with how other replacement effects, such as Zhonya's Hourglass, function in Riftbound (see RiftJudge FAQ #8617).