Britsimon,I asked some others about it. I remember having been confused about that section in the past. If you look at the section before that however, this is apparently superseded by the "green card test". Also if you see later the section under dual status, it says:
"
Residency starting date under green card test. If you meet the green card test at any time during a calendar year, but do not meet the substantial presence test for that year, your residency starting date is the first day in the calendar year on which you are present in the United States as a lawful permanent resident.
If you meet both the substantial presence test and the green card test, your residency starting date is the earlier of the first day during the year you are present in the United States under the substantial presence test or as a lawful permanent resident."
In other words, I think your tax expert has it wrong
Edit to add: you are considered an LPR as soon as your immigrant visa is stamped on arrival. It does not depend on when you receive your physical green card (I think you know this, but just in case some others reading this don't.)