I heard someone say DW will be the Primaris Codex. But what does that mean? Don’t we already have Primaris in the SM Codex?
Means Primaris will be in the Deathwatch book, which should come as a surprise to absolutely no-one, an account of the fact we've been able to take Primaris units in DW since last August.
No, it means that the DW Codex will have stuff that works really well with Primaris units. The anti-xenos sniping Stratagem in combination with Hellblaster for example.