That is not entirely correct. Please see the IRS.gov for this (2017) rule:
Separate return.
If you file a separate return, you can take an exemption for your spouse only if your spouse had no gross income, isn’t filing a return, and wasn’t the dependent of another taxpayer. If your spouse is the dependent of another taxpayer, you can’t claim an exemption for your spouse even if the other taxpayer doesn’t actually claim your spouse's exemption.
