Whoever you spoke to has a lower than average IQ and informed you incorrectly. A 90 day Non-O based on retirement 100% CAN be applied for inside Thailand at any immigration office. You'll need 800k in a Thai bank account and you're passport.
No, it does NOT have to be a thai bank. Money from your home country needs to equal 500k thai baht. Different embassy's require different documents though, some want 3 months, some want 6 months of statements. Where are you applying?
You can apply for the initial 90 day Non-O inside the country at an immigration office. You'll need 400k baht in a Thai bank account (but there's no time requirement on it).