Irresponsible

I do not think that the matter at stake here are the guarantees that can be expected from FOSS, rather more of a marketing thing. If a big company attaches its name to a product (irrespective of the guarantees it provides) and the users are disappointed (totally irrespective of whether correctly or wrongly), then that is a damage to the company that from that moment on will be perceived in a different way (either slightly different or significantly different), possibly orienting further purchase either directly (by the disappointed ones) or indirectly (by those who heard the message by grapevine communication).

So the point is whether providing some 6 month advance to the distro EOL would have costed more than such damage. Which is a computation difficult to evaluate from the outside as it necessarily takes into account multiple factors, including the situation the company is in, current perception, resilience of positive perception, etc.

Another point is whether it is not overly optimistic to attach the brand name to experiments made for their internal value.