I.e., Garuda sells seats on Delta-operated flights to/from the US (between Haneda and LAX/SEA), but Delta does not sell seats on Garuda-operated flights.
You appear to be correct about Garuda not currently flying to the US - I'd flown them out of LAX previously (and I know they used to go Bali/Honolulu) but apparently had to stop.
However, Delta does sell on Garuda flights. You can book Delta Seattle/Jakarta and the second leg is operated by Garuda.