I see he appears to be in the San Francisco area. This means it could well be Douglas fir. More likely Doug fir, I think, than 2x material made out of cedar.
But I agree with the spirit of the original. It's just a raised planting bed. Use it till it falls apart, then make another, using the lessons you learned the first time.
You might be surprised at regional variations in dimensional lumber species.
2x's are commonly eastern white pine in many parts of the country (or "SPF" -- spruce/pine/fir, no promises, but not douglas fir). DF is sometimes also available, as is SYP. Both are more expensive than WP, slightly.
4x4s are always DF, in all locations that I've noticed. No doubt due to its superior straightness and strength.
Also curiously 2x3 availability (always WP, afaict) varies greatly. I've had lumberyard guys stare at me like I was crazy for asking where the 2x3s were. These weren't new kids, either.
All of these variations might be influenced by local building codes, in addition to regional distance-to-market.
As for the species in the photo... Could be cedar, but the grain looks pretty tight -- another possibility would be redwood, which is appropriate for the use, and is available at west coast lumberyards. Can't get redwood on the east coast except at specialty yards, and even they generally don't stock it.
You are right, redwood is a distinct possibility. Non-heart or mixed heartwood/sapwood redwood can look like what is shown. Although the sapwood has very little resistance to insects, at least in southern CA where I build stuff.
But I agree with the spirit of the original. It's just a raised planting bed. Use it till it falls apart, then make another, using the lessons you learned the first time.