I believe that is exactly what they are doing. By not signing any of the OL that are out on the market right now, they are keeping all the options open to draft BPA.
To many on here keep saying, sign a tackle now, and that allows you to draft a guard in round 1. What happens if by odd chance Lane Johnson is still on the board, but neither Warmack or Cooper are on the board. Do we still reach for the next best guard?
Right now, there are 3 FA tackles on the market. Most of them, and quite likely all of them will STILL be on the market after the draft. And possibly a few more as well.
The smart thing to do, is wait and see who the best available talent is at #18. If after the first 3 rounds, you don't have a OT, DT, or OG then after the draft you find a short term guy to bring in to fill that hole. There isn't an OT, OG, or DT on the market that is more than a stop gap player. The only way to truly fill any of those holes in threw the draft.