Your compressor will work, just put the retainer on and compress away.
It will take a long time for you because of this tool.
The reason is this tool was made because Chevy engines burn up valve guide seals. So what people do is compress one spring at a time on the engine and install new seals without taking the engine all apart.
Racers use this compressor to change out a broken valve spring at the track. And I think other people use this type compressor to remove break-in springs and then install the heavier pressure springs. This way they do not wipe out their new camshaft when they first start up a newly built engine.