InStr function returns pseudo True/False?

I have been playing with the Instr function (Excel 2003), and it appears to behave as though it's returning a True/False value.
For example, the following will print "1":

If Instr("abc", "b") then 
debug.print "1"
else: debug.print "0"
end if

I said "pseudo" above because if the code tests for "True", it doesn't find it. The following prints "0":

If Instr("abc", "b") = True then 
debug.print "1"
else: debug.print "0"
end if

What's the go? Safe to use it like that (boolean)?