That is really odd. What are they doing? Hiking to the wedding spot?
I’d wear wedding appropriate clothing determined by time of day. I’m not going to lower my standards.
What is the venue and location? Like if it's on a farm, don't wear heels that will sink into the ground. If it's in the mountains, bring a sweater. That sort of thing.
Where is the wedding? Is it on a beach in the summer in a high humidity area? One of the trendy farm settings? Location would drive the definition of comfortable in my mind.
I’d wear a casual maybe jersey knit knee length dress, maybe a simple sundress knee length or midi, something like that. Flats or heeled sandals. Not pumps of any kind.
Men will probably wear jeans and an untucked shirt or polo, or chinos of some kind. Comfy shoes some will probably wear clean sneakers. Like going to dinner in any city.
That’s what I’d think of as casual but appropriate for a wedding. But with that language I bet someone shows up in a hoodie or lulu or a t shirt.