Hi Larissa,
It is correct. You can only receive one pension from the OASI (AHV). Entitlement to a widows pension from the OASI ends when you reach retirement age and begin receiving an OASI old-age pension.
The same is true for widow's pensions from occupational pension funds (BVG). These also end when you begin receiving an old-age pension from your pension fund.
The only Swiss social pension which you continue to receive for the rest of your life is the widow's pension from social accident insurance (UVG). You may receive this if your spouse passed away because of an accident. You keep receiving this pension even after you retire.
I hope this answers your question.