It’s true. Many Big, Enterprise companies will have informal and even formal policies limiting the vendors they can buy from to “established companies”.
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.