I think (but I'm prepared to be corredted) all 6.1s have indirect monitoring as standard - my Startline certainly does and rheess no lower spec option that that - and direct sensor monitoring was an option on some models.
I would suggest it's a fairly pointless option. Indirect monitoring is extremely reliable as its essentially just an extra bit of software that reads signals from the wheel speed sensors.
It's also very sensitive - mine recently pulled my trousers down and gave me a good spanking because it had detected one of the rears was 2psi blow then other three boots, which I thought was pretty good. I reset the tyre pressures at the local garage and within a few hundred miles it congratulated me and the warning cleared.
It's so reliable and sensitive id beg to question why anyone would want troublesome wheel sensors at all?
That said, you had a flat and had no warning...which completely undermines what Id believed in my first paragraph...yet my basic spec Startline has it....?