Bike fit question: does exposed seat post length matter?

Spa Cycles are probably using the 'old' size measure used still by a few manufacturers - like Colnago - of giving a size which is the seat tube centre to centre. A 52 in Colnago is about a 56 in most other bikes.

What the geometry comparison referred to elsewhere shows is that the Elan will feel a bit longer, but with a slacker head angle and longer seat stays, it will also feel calmer and more relaxed.

As for the seat post question, you can correct the seat-bars position with a setback seatpost or a shorter/longer stem. More exposed seatpost will mean a more comfy ride in my experience. It's what I notice most when riding an old steel bike, the short exposed seatpost is quite rigid and you feel the thumps from the back wheel more.

Some bikes are deliberately designed to encourage more exposed seat tube to increase comfort and compliance. It would be perfectly possible to design two bikes with identical effective geometry but where one will have more exposed seat post than the other. 

What matters for a bike fit is the relationships (angles and distances) between the contact points - i.e. the relationship between saddle, pedals (bottom bracket) and handlebars. What goes on in between is irrelevant for fit (although is relevant for other purposes such as comfort and handling). 

It looks to me like the Elan is designed to be more of a touring geometry than the Defy (i.e. designed for a more upright riding position). It might be possible to get the geometry closer to your current Defy by changing a few components but I suspect if you want to exactly replicate the fit, you might want to look at "sportier" frames. 

I think this discussion demonstrates why elderly gentlemen like myself focus on effective top tube rather than tis stack and reach stuff, particularly the latter. The relationship between saddle and bottom bracket is determined seat tube angle, and is not really critical as time trial and tri bikes seem to work with very different proportions.

I dont believe that a 52 frame will have anything like the top tube length of a m/l Defy.

This is a very useful resource:

https://geometrygeeks.bike/compare/spa-cycles-elan-2017-54,giant-defy-advanced-1-2020-ml/

Not sure I have selected the correct sizes and models for you but you can obviously do a more relevant comparison if needs be.

According to Spa's geometry chart the size 52 on the Elan is a little taller than the Giant Defy M/L (51.5cm) so you would actually have a little less seatpost showing. 
Being that the size 52 is the second smallest in the range I can't believe that the chart is right.

Long seat posts are common these days. There needs to be enough post in the frame to hold it securely, but the Giant dfuse post is meant to be long and wobbly - the give allows for more comfort.

I ride a ML Defy Advanced 2017 and am 5'11 and a bit with 31" inside leg, so a bit long bodied.

I don't ride slammed, I've got all the original spacers in place. Trouble with dropped top tubes is that frame dimensions don't help so stack and reach should be your guide, comparing the relative ones but bear in mind that stem and handlebars are not included. The long body would tend to push you to a larger size, getting the saddle height right is easy enough on a modern bike so length is the more critical dimension.

Geometry isn't my strong point but stack and reach are measured relative to the centre of the bottom bracket. So seat tube angle will be relevant to where your saddle sits fore and aft, but not seat tube length.

My guess is that we can rely on stack and reach because seat tube angles don't vary that much, and so you can fine tune with saddle setback to dial out any difference - up to the point that you're no longer in the right place over the cranks.

But I am no expert on this!