First, I always over kill. IOW if the rock looked to weigh 200# I'd use a 500# strap. I'd probably get the 1000# straps, any rock bigger than 500# and I doubt I'd try to winch it off the trail anyway, dragging that much dead weight with a winch would be difficult.
You are talking about safety factors here. If your strap is rated for 20,000 lbs and you are pulling 5,000 lbs of resistance then you have a 4:1 safety factor. In the lifting industry a 4:1 or 6:1 safety factor is not uncommon. I'd go more than the 2:1 you state for a 200 lb rock.
Second, unlike a recovery strap that is intended to stretch and recover the cables and straps used in winching really don't flay around all that much when stuff breaks or slips. Specially when it is a relatively light object ie not a truck. BTDT.
Steel cables do stretch and do recoil - violently. That is why many use synthetic winch line.
When you mention recovery straps versus straps used in winching you might be thinking about nylon versus polyester straps. Nylon typically stretches more than polyester. Snatch straps are nylon, tree straps (proper tree straps) are often polyester. Winching should be done in a static environment with as little stretch as possible.
It "looks" like a decent strap. It is rated with a break strength (probably average) of 27,000 lbs at 3" one ply which = about 9,000 lb per inch width which is about the norm. The problem with buying some imported recovery products is you can't see the issues. With cheap imports issues can (but not always) occur with the bulk manufacture of the webbing, or in the assembly. Your standard deviation of break will also probably be wider than a higher quality strap, which means the spread of straps breaking will be wider than nicer straps.
This strap is also polyester, and probably lower stretch, so it would not be wise to use for a snatch strap.
I've used straps like this before - and they worked - BUT - when dealing with load bearing recovery gear I suggest running quality equipment and deploying that equipment correctly.
And now I digress... Re: correct deployment - I had a customer come in the shop recently and talk about their 2 ply 6" strap they use to snatch people out of holes. a 2 ply 6" strap is effectively a 12" 1 ply strap. 12" x 9,000 lbs/in = an assumed break strength of 108,000 lbs. If the strap were nylon, and had an elongation at break of 8%, and you could only load it to 20,000 lbs during a snatch that would = an elongation of around 1.5% which = a bone jarring snatch. While the strap probably won't break, you are gonna feel that impact and that is not the correct use of recovery gear.
So I cut off the legs which make great sleeves for cables and straps when rubbing is a concern. Yea for Recycling!) 
