You are right, this is BS.

I recently researched this and Germany's grid is quite "smart" (the oldest technologies involved, such as DECABIT or VERSACOM over PLC, very much predate the term "smart grid" but whatever) and power plants and households are connected for production and load control. Power plants are required to participate but households can use a load management system for water tank heating (the basic premise is that specific frequency impulses are sent over the power grid for primitive (originally relay-based!) logic in DECABIT meters to switch depending on the assigned device group, and meters count in lower-price mode while the load is activated for a guaranteed number of hours each day; you can manually override the switch for expensive on-demand water heating) and/or HVAC (here, a smart thermostat is usually used that gets real-time energy prices and decides based on its temperature range settings if it saves money to run heating/cooling).
People in Texas apparently hate this (muh freedom), and look how reliable their grid is!
Anyway, solar, unlike coal or nuclear, is absolutely capable of going off-grid if necessary. There is an MPPT system in their inverters that usually works to operate the panels at the optimal voltage & current so that it can suck the most power out of them but it can be overridden to work at below 100% efficiency, or even 0%. This will cause the panels to run with no current draw and get about 20% hotter but they are designed to withstand this. Similarly, wind turbines can be braked, water can be passed outside turbine shafts and so can pressurized steam if you really need to cut production quickly. Still, this is an emergency condition, it is preferred to use pumped hydro (responds in 1 minute, limited capacity) or batteries (respond in seconds, very limited capacity) or lower coal/gas-based production (responds in 3-20 minutes for as long as you wish) or load-side management to regulate the grid, as it wastes no power.
The system is very complex and robust, the frequency (the variable most dependent on production/load balance) only dips below 49.8 Hz about once per a few years (the emergency value that was reached in February 2021 in Texas and can only be sustained for minutes before total blackout is -1% from nominal (49.5 or 59.4, respectively) and has never been touched in Europe's modern history).
(You'd think it would be voltage what falls in case of too little power but it can be readjusted quite easily with switched transformer taps and, oddly enough, reactive power management (connecting a few capacitor/inductor banks to mains) when necessary, however frequency control is the difficult part.)

The grid needs the supply and demand to be balanced for the power to be stable. Otherwise you get fluctuations in voltage and frequency which are both bad for anything connected to the grid.

There can absolutely be an oversupply of energy. We need to either find ways to store that surplus energy, or use it for something positive like desalination or carbon capture.

Free energy should mean algae production

This is a jump I'm not understanding. Do you mean that energy having no cost would mean that the electricity generated can be used to make algae? Or that it's a byproduct of?

Germany actually spends money to stop Denmark producing power on windy days when prices get too low. Instead we could be making hydrogen and storing it in so many creative ways

Its 'free' anyway so there should be no concern about how much of it is lost in conversion

Could you elaborate how to grow algae with electricity?