It's still context-free, the reason is because by the time you hit the '=' symbol you already know whether you're in a <Statement> or a <Compare Exp> production rule, based on the preceding symbols (namely the LET keyword), based on these excerpts from the BASIC EBNF:

    <Statement>   ::= CLOSE '#' Integer
                    | DATA <Constant List> 
                    [...]    
                    | LET Id '=' <Expression> 
                    [...]
                    | Remark

    <Compare Exp> ::= <Add Exp> '='  <Compare Exp> 
                    | <Add Exp> '<>' <Compare Exp> 
                    | <Add Exp> '><' <Compare Exp> 
                    | <Add Exp> '>'  <Compare Exp> 
                    | <Add Exp> '>=' <Compare Exp> 
                    | <Add Exp> '<'  <Compare Exp> 
                    | <Add Exp> '<=' <Compare Exp> 
                    | <Add Exp>