“Filters are optional” is that not all filters apply to all order types, as mentioned by another user.
The PRICE_FILTER defines the price rules for a symbol
Not all order types require the parameter price. (I.e. MARKET orders), so in this scenario PRICE_FILTER does not apply to that case.
The TRAILING_DELTA filter defines the minimum and maximum value for the parameter trailingDelta.
Meaning this would only apply to orders that use the trailingDelta parameter, also known as Trailing Stop Orders.
In addition, there is a possibility not all filters in the documentation will be configured to all symbols. The documentation just lists all possible filters that can be configured, so if the users sees this as an error they can look it up and understand what they need to do in order to pass it.