The tone timing is in the audio sample is weird and out of spec. Minimum tone duration is 65 ms, and minimum pause duration between tones is 65ms. [1] That example has much longer tones than pauses, and there seem to be some back to back tones. The article says it's taking too long to send the data, and that's why.

If you send tones 100ms long with 100ms pauses, a conservative rate, you can get 5 digits per second. That's about what "redial" on my phone clocks at.

[1] https://www.etsi.org/deliver/etsi_es/201200_201299/20123502/...