That should indeed be correct, although as others have noted, those staffing the access points to the lounge sometimes seem to get it wrong. I'd hope at least that someone working in the Concorde Room and dealing exclusively with F passengers (and of course the occasional Prem or CCR Cardholder) would be more clued in.

If there's enough time on the return (OP mentioned already that outbound is a shorter connection) then consider asking YouFirst to book a cabana as well (subject to availability). Not only is it nice to have your own dedicated space/shower/etc., in the unlikely event that someone tries to tell you you're not entitled to the lounge, you can point out that if that were so, then how on earth would YouFirst have already reserved you a private cabana inside the lounge!