It’s true. Many Big, Enterprise companies will have informal and even formal policies limiting the vendors they can buy from to “established companies”.
But …
They all have an exception for (x) new, emerging vendors that (y) solve a problem no established vendor does. Or at least, no established vendors solved for a certain use case.
And they even have special, 50-100+ page policies around how to deal with these cases. In particular, what levels of additional risk to take. Typically, these 50-100 page policies all boil down to certain additional risk because acceptable in (x) non-core business processes where (y) the ROI is perceived to be very well.
So yes, what you are hearing is true. But — also — there is an answer to this chicken-and-egg scenario. Of course there is.
For you, it’s making your product much more valuable and uniquely good at at least 1 important thing these Big Guys need. Then. They can crack open the Exceptional Manual.