This correct. Text is simply - well - text.
But, you should be able to store a duration as a text string and parse when performing evaluations. The following is PTP (precision time protocol) which is used in the slow-mo video and broadcast industry.
00:00:01.150230 (i.e., 150.230 milliseconds)
The first three digits after the decimal is thousandths and PTP brings in another three digits of precision. For swimming events milliseconds is probably enough.
While Coda doesn’t support PTP or milliseconds natively in time or duration fields, you are free to employ arbitrary formats in text. The challenge comes when utilizing the text and performing computations on them. But, there are plenty of formula functions to help.
You might also consider using separate fields to store the numeric parts of the value and then use a formula to join them for computations/sorting and/or display.