[Phil]

With a strong 4-5, its perfectly safe to rebid 2♦, since we are in a forcing sequence. Its not about having multiple weak bids, but simply showing the long diamond. In vanilla 2/1, I would expect any hand with at least five diamonds to bid this way, and for 2♠ to show less than five diamonds (unless, perhaps a 5-6), although 20 years ago, two level reverses definitely showed extras, because 1♦ - 2♣ frequently wasn't a GF.

In the posted auction, 2♣ obviously isn't GF, but we are still in a force at the two level.

I realize that a lot of this sounds like something out of Alice in Wonderland…

[beatrix45]

They tell me that even with 2/1, with an overcall, you are back in the SAYC jungle. If so, then 2♠ is just the cheapest and most convenient rebid. Seriously, you can't Pass. 2♦ shows five♦ and a one suited hand.

[akwoo]

The 2♣ bid generally denies 4 spades (since a negative double is available).

Hence 2♠ is a reverse.