From https://www.worldtravelguide.net/guides/asia/bangladesh/money-duty-free/:
All foreign currency exchanged must be entered on a currency declaration form. Hotel bills must be paid in a major convertible currency or with traveller's cheques.
Anything out of the ordinary (to a westerner) if one gets local currency from an ATM and pays with that?
When you buy local currency from the ATM you are exchanging currency, selling your currency and buying Takas.
Generally in countries with currency controls you should save the ATM receipts so you can convert unused local currency back to hard currency when leaving (probably with an awful rate and commission, but as you can only legally export a negligible value of local currency according to your link, you will have little choice).
Those unofficial sites often have information that is out-of-date or outright incorrect so be sure to check around if it might matter.
