I think the expert is partially incorrect.
You can amend to MFJ, because that was always an allowed option. However, when filing MFJ, you must report and pay US tax on all your spouse's worldwide income, even from jobs outside the US, and even though it might be taxable in another country. (At the time, you would have been required to apply for an ITIN for your spouse, now you can use the SSN.) Whether this is a good idea or not probably depends on whether your spouse had taxable income in those years that was not previously declared to the IRS.
However, the expert is correct that certain specific tax benefits that might be available to US persons (citizens or green card holders), will not be available to your spouse or child, unless the SSN was issued before the deadline for a timely filed tax return (meaning April 15, or October 15 if you had an extension).
That means that even if you file amended returns to file MFJ, you won't be eligible for EITC, or the Child Tax Credit, or the pandemic stimulus rebates for your spouse or child. Also, Turbotax won't know they aren't eligible, so it will show those things as part of any additional refund you might claim even though the IRS will remove those items and recalculate any refund before it is paid. So it will be a bit trickier to figure out how much you might actually save by amending those past years.
