Is it trademark infringement if startups use well known brands in their slogans or high concept pitches?

Be aware that the reproduction of a trademark, even if it is a slavish ("verbatim") reproduction of a well-known brand, will constitute an infringement - legally speaking - only if the trademark that is so reproduced is intended for commercial use, regardless of the law to be applied.

If these start-ups simply illustrate their business model by simply referring to the popular brands without using them to offer for sale and sell identical and/or similar products or services designated by the said trademarks, they cannot be prosecuted for infringement.

However, one can imagine that other grievances may be objected as misleading advertising, damage to the image (eg the use of a car brand by gangsters in a video ...) or parasitism in some cases, but this is another topic...