Parkey Valby swam competitively for 14 years.
When she is cross training, it's very intense.
Don't believe for one second that she rolled off the couch, started training 2-3 times a week, and, bam, she is a sub-15 5ker.
It's a body of work for 15+ years of hard aerobic training.
The body does not care where the aerobic stimulus comes from, you'll still get the benefits if you built it up over many years. It falls in line with what Gjert said in an interview about early-life training.