Is this solid advice?
No, this is speculation, this is not solid advice. It is only solid advice if part of your income and/or expenses (current or expected) are in USD. Then you might want to hold part of your cash as USD. Otherwise all of your cash should be in your local currency.
Would it be solid advice depending on the currency conversion costs?
Even less. All transactions have a transaction cost. It is not solid advice with zero transaction costs, and it is even less solid advice with positive transaction costs.
Does this carry the same risk as normal forex speculation or is it lower risk because it involves (seemingly) stable economies?
What you consider as "normal forex speculation" is the key here. For me, normal speculation is buying something and selling something else to take advantage of supposedly known exchange rate changes. So, according to me, this is exactly as risky as "normal forex speculation", because the advice is just that -- normal forex speculation.
However, there are people who have highly leveraged forex positions. They take a huge loan in one currency and then sell that currency to buy something else. This advice does not involve leverage. So, it is less risky than a leveraged forex position (which I would never ever do).
Something to think about is the time frame, too. The longer the time frame, the larger the risk. Those who do forex speculation often do short term speculation. This advice, a form of long term speculation, is more risky than short term speculation.
So, to summarize:
- The larger the leverage, the larger the risk (the advice involves no leverage)
- The longer the time frame, the larger the risk (the advice has a long time frame)