I've just had the same "subject to availability" discussion with the Westin Grand Vancouver on a current (otherwise delightful) stay. I'm Lifetime Gold.

William, I've emailed you the particulars. Let's just say the front desk rep was adamant I was wrong (repeatedly) when I requested late checkout, told me I couldn't request it until the night before or day of departure, and that it was only "subject to availability." When I called back to talk to the manager on duty, she said she would offer me the late checkout even though it was too early and was still "subject to availability." When I read to her what was on SPG.com (and when she acknowledged they were not a resort or conference hotel), she said that their staff expert on SPG had told them otherwise.

I suspect a misunderstanding that might be gently corrected.

If the hotel really can't provide it due to capacity or other issues, I'm certainly willing to be flexible. But this was stated to me as standard policy.