There is no single "relational algebra". They differ even in what a relation is. Codd originally defined theta join as taking a binary operator (the theta) & two attributes. That is what people usually mean by the term. From Codd's 1992 book The Relational Model for Database Management Version 2:
RB-14-RB-23 The Theta-Join Operator
The theta-join operator employs two R-tables as its operands. It
generates as a result an R-table that contains rows of one operand
(say S) concatenated with rows of the second operand (say T), but
only where the specified condition is found to hold true. For brevity,
this operator is often referred to as join.
The condition expressed in the join operator involves comparing each
value from a column of S with each value from a column of T. The columns
to be compared are indicated explicitly in the join command; these columns
are called the comparand columns. This condition can involve any of the 10
comparators cited in the list [...]
(Often the term is used for an operator like that but where a value can take the place of an attribute.)
But Codd contradicts himself there, because there are "two operands" but also "the specified condition". Later he contradicts himself again, saying there are a bunch of joins collectively called the theta-joins, and equi-join is one of them:
RB-25 The Natural Join Operator
As described in the last section, an equi-join generates a result in
which two of the columns are identical in values, although different
in column names. These two columns are derived from the comparand
columns of the operands; of course, the columns may be
either simple or composite. Of the 10 types of theta-join, equi-join
is the only one that yields a result in which the comparand columns
are completely redundant, one with the other. The natural join
behaves just like the equi-join except that one of the redundant
columns, simple or composite, is omitted from the result.
The textbook you quote uses the term to refer to a different thing, where theta is a boolean expression combining comparisons. That is reminiscent of SQL INNER JOIN ON complex conditions. For Codd it is:
RB-24 The Boolean Extension of Theta-Join
Observe that your quoted textbook's theta-join is not an algebraic operator (mapping argument values to a result value)! It is a language non-terminal (in which a comparison expression does not denote a value). The textbook doesn't actually define sufficient algebra operators to implement its algebra-style query language. It's alleged set of basic algebra operators includes select but it actually only defines that as a language non-terminal since one operand is a condition expression.
(This sort of sloppiness is typical of relational database textbooks. There is a culture of fuzziness & confusion in the relational database field. Which, like the field, was begun by Codd.)
