It seems odd to, in a security context, say "The payload of the HeartbeatRequest can be chosen by the implementation", giving a huge amount of freedom to implementers (apparently it was determined that "something more elaborate" might extend to an implementation needing to send a full 64K of chosen payload data) without further investigating whether any particular choices made by the implementation might have consequences for the security of the protocol.
Why allow variable length payloads at all? Why allow more than about 128 bits of data for distinguishing responses? I guess you might argue a client might want to disguise the nature of these heartbeat packets from an eavesdropper, so they need to be variable in size - that's reasonable, but if it's a requirement, then surely you'd want to comment on the security risk of using a "simple sequence of numbers" rather than "something more elaborate". 64K of data, though? That seems like a lot of leeway to give an implementation - considerably more than enough to disambiguate heartbeat packets; implementations are going to keep 64K of data around and compare it byte for byte to check whether the echo they got back matches the payload they transmitted earlier?
I really don't mean to ascribe malice here; I tend to assume this is just a bug, and that the payload for heartbeat packets gets to be up to 64K because once you've decided to make something a variable length buffer, you're probably choosing between having a one byte or a two byte field for length, and 256 bytes seems like it might be too small so you figure 'why not make the length field two bytes?' and then... oops; someone makes a buffer overrun and suddenly we're faced with 64K at a time of data being ripped out of any old server's heap.