It's more complicated than that.
There are more than 30 brands in the Marriott Bonvoy program. Nine of them provide lounge access to Platinum Elite and above.
At five brands (non-resort properties only) -- JW Marriott, Marriott Hotels, Delta Hotels, Autograph Collection, Renaissance Hotels -- guaranteed lounge access is how the breakfast benefit if provided. If there's no lounge or the lounge is closed, the elite member can pick breakfast in the restaurant or 750 points daily.
At one brand -- Courtyard (only outside the United States and Canada, and only if there's a lounge) -- guaranteed lounge access provides a breakfast benefit (at a brand where that's a rarity).
At three brands (resorts included) -- Sheraton, Le Méridien, Westin -- guaranteed lounge access means access to the lounge if there is one and it's open, but there's no alternative otherwise. Breakfast in the restaurant is a Platinum welcome gift choice, not a function of guaranteed lounge access.
The front desk agent at participating properties will normally mention the lounge and its hours at check-in, but sometimes it's necessary to ask.
For all the details, exclusions, limitations, etc., go to
Marriott Bonvoy Loyalty Program Terms & Conditions and search for GUARANTEED LOUNGE ACCESS