You're right. My natural inability to get things wrong took over there for a minute when I wasn't looking. I mean, they write it as TU.
It's not so weird that they write it that way - it's just that those characters are exceptions to rules. For example, if you learn that かきくけこ is KA KI KU KE KO, then you learn たちつてと as TA TI TU TE DO, despite the fact that the sounds are a little different. It just comes from following a rule and not knowing actually how to pronounce what you're writing.
It's the fault of the Japanese English Education being based all on reading, writing and grammar as opposed to talking and communication. Grr.
But yeah, I do find it cute when some kid or a cute girl bastardises the language without realising it