Additionally, I'm guessing that a Titan 737 has more than 19 seats?
Yes, but I suspect the counter-argument that an ATOL-exempt travel organiser would make is that temporarily they cannot adhere to letter of the 'Small Aircraft' restriction because of the CAA's own actions...
The other wriggle-space is provided by the wording 'on flights on aircraft with a maximum capacity of 19 or fewer seats available for passengers'. It doesn't specify *certificated* capacity, just the maximum made available.