You're missing the point - if the signal is green how can the driver be sure whether the guard checked it or not? Yet if the guard doesn't check the signal surely it's a safety breach whatever the signal shows, not just if it's red and the driver refuses to move off and reports the incident. If the guard not checking the signal when it's green isn't a safety breach but the service has to be cancelled if the guard gives two buzzers and the driver ignores them then the rules make no sense.
The driver can I suppose never be 100% sure that the person ahead of them in the chain (the guard) has done their job 100% correctly.
I don't think you quite appreciate the rules surrounding dispatch, signal aspects and crew communication and the seriousness of this.
If you think that a Guard giving 2 on the buzzer is a non-issue because the driver was vigilant and did not act upon them then you are very much wrong.
Giving 2 against a red is an operational incident and is defined, managed and reported as such. The outcome (ie. what did the driver do next?) does not have an impact in downgrading or overlooking the issue.
It cannot be looked over just because 'this time' it did not cause a SPAD. That would be like glossing over all TPWS incidents for the same reason, or ignoring the release of doors not on the platform because this time nobody fell out.