Solarwinds DR Deployment Additional Pollers

Yes, that is precisely how a multi-subnet failover configuration would function. Now not everyone has backups for their APEs, and chooses instead to monitor their most critical systems from their main poller. In the event of a disaster, the main poller fails over to the DR location and continues to monitor critical items, but the tier 2 stuff monitored on the APEs is not. That's one approach some users take, while others ensure there is full redundancy for everything. The decision usually comes down to budget and necessity.

if i setup multi-subnet HA for APE , rather than for Main polling engine,, During a DR scenario does my APE HA continues to run,despite my main polling engine is down aLTeReGo 

Yes, if you were to lose you main poller and APE, but your APE had an HA backup then polling would continue. You would still lose access to the Orion web interface and Alerting though in that scenario, but there should be no gaps in your data for things polled/managed from that APE during the outage.