Yes it does. Either a marking alone or a vertical sign alone could be sufficient for a mandatory disabled bay (backed by a TRO) if the authority deemed that to be “adequate information”.
TSRGD 2016 stops you placing either sign or marking without a TRO, so it seems there is no legal means for placing the ‘advisory’ marking that clause 9 of the 2009 DPPP Act you mention refers to. It only permits an advisory bay as a temporary measure whilst you are going through the order making process, so ideally needs to be similar to the final signing, not something to be removed or altered when the TRO comes through.
The main thrust of that Act seems to me to force authorities to review their existing ‘advisory’ on-street disabled bays and either make them mandatory or remove them.
I’ll raise this as an issue with Transport Scotland. Thanks for pointing it out.