I think the TCAS comments are a red herring. We get TCAS alerts all the time from ground or circuit aircraft. My guess is it’s highly likely both aircraft knew the other was in close proximity.
Assuming the landing aircraft had TCAS and the departing one had a transponder on. If the landing aircraft expected the departing one to remain on the pad every landing would give a TCAS alert. This would render an alert in this situation ineffective.
My experience of TCAS is it is a good alert to get eyes out for an aircraft that is not known to be there however it’s not suitable for separation in helicopter operations particularly close proximity.