It is a myth. High load has nothing to do with it. However if you configure apache wrongly, it will serve .php files as text. Only connection to load is if you have one broken server among N proper ones, a number of times that the broken one is hit depends on load - I.e. on low load, it may be configured so that it is never hit at all.

I guess the story about apache serving php source under high load came from the idea tha Facebook is a high load site (true) and they couldn't just have made as obvious screwup as misconfiguring a production server (false) so it must be apache/php bug that happens only in high load sites (false)