McLaren argued that the conditions meant that Lando gained zero advantage by not fully navigating the last corner within track limits and actually started his next lap at a lower speed because of it.
There is no rule that the next lap is deleted, it is part of the Race Director's Event Notes specific to each weekend, this weekend says:
13) Track Limits
In accordance with the provisions of Article 33.3, the white lines define the track edges. During Qualifying and the Race, each time a driver fails to stay within the track limits, this will result in that lap time being invalidated by the Stewards.
Whereas, for example in Bahrain where the rule applies, it says:
13) Track Limits
In accordance with the provisions of Article 33.3, the white lines define the track edges. During Qualifying and the Race, each time a driver fails to stay within the track limits, this will result in that lap time being invalidated by the Stewards. Additionally, each time a driver fails to respect track limits at the entry or exit of turn 15, will result in that lap time and the immediately following lap time being invalidated by the Stewards.
So the Steward's were wrong to delete it, and McLaren didn't need to make any such argument. If they had, I think it would have been overturned on appeal because the rule doesn't include any clause about gaining an advantage when it is introduced.