I've written this before, and I'll briefly repeat.
This is not technically correct. However, I believe they're referring to the appropriate fare class bucket (eg: I for Business) for WP to request. Sale fares *generally* book into I inventory for example. However, flights can have I inventory but no sale fares simply if none are filed for the route in question.
So one can look at a flight via the booking engine and see nothing "on sale" but there could well be I inventory available. This is why a service like EF, or ideally, access to the GDS, is so helpful to tell.
Again, agents may just be using "sale fare" phrase to simplify the explanation for punters - and I would accept it on that basis (but then again, I look on EF first anyway, so I have a fair idea before requesting).
I don't think it's unreasonable for agents to go this route. It's fairly niche for non industry people (or geeks like me) to have awareness of inventory buckets, required buckets for various status levels to request release, and the like - and fin practical purposes, it's not a totally inaccurate explanation.