I do not get a syntax error with the statement. Note that the first statement is TSQL dialect and the use of semicolons as a statement separator in TSQL was not supported until 17.0 and requires a database upgraded or created using 17. You can query the table syshistory to confirm the version of software used to create the database and whether the database has been upgraded and which version was used. That said, I also do not see an error with this statement using a v16 running on the same version of SA17 as you reported.

Interestingly enough, the first statement (used with MyTable instead of TABLE as table name for obvious reasons) returns 'Transact-SQL' both with 17.0.10.6175 and 16.0.0.2798:

select sqldialect('SELECT MyCount = count() FROM MyTable WHERE TableID = ''58693'';')

This is a message for future lurkers...

Sorry to be so late to the party, but...

SELECT count() as MyCount FROM Table WHERE TableID = '58693';

absolutely positively DOES NOT WORK on SQL Anywhere 16...

Could not execute statement.
Syntax error near 'Table' on line 1
SQLCODE=-131, ODBC 3 State="42000"
Line 1, column 1
(Continuing after error)

...or SQL Anywhere 17...

Could not execute statement.
Syntax error near 'WHERE' on line 1
SQLCODE=-131, ODBC 3 State="42000"
Line 1
SELECT count() as MyCount FROM Table WHERE TableID = '58693'

...with or without the semicolon.

Details matter, especially when exploring the absurd differences between Transact SQL and Watcom SQL.

Details like copy-and-pasting actual code when asking questions on this forum.