Haha, that was me. Sorry about that, I shouldn't have used such a phrase on the ELL site.
You do understand the meaning of the phrase correctly (apart from the "opposite meaning might be equally likely" part perhaps) - it does mean something along the lines of, "and this far-fetched, but I'll say it anyway". As such, a reach is closer to "something you can achieve because it's still within reach", though "far-fetched" fits better.
In terms of definitions, I would say this definition of the verb to reach (then changed to a noun) would be closest / most applicable:
To make an excessive effort, as in drawing a conclusion or making a joke; overreach.
To strain after something.
Here are some other sentences with this use of "to be a reach", taken from COCA:
Bit of a reach for a first-rounder, but Texans like him.
Now to the Gulf. It is much too far a reach to suggest that peace is in the air, but certainly to the recent...
That's the gamble we're taking. I know that's a reach.
Couldn't we say -- and I don't think this is a reach to suggest this -- couldn't we say that social engineers -- liberals -- are...
...guidance counsellors suggested when applying to colleges: Make two submissions that are a reach, two that are in range, and two you would consider.
I find this last example is particularly helpful in exemplifying the meaning: counsellors recommend that high school students apply to 6 colleges; the first two are the ones you really want to get into, but you doubt you have a chance (but you might be lucky and get in, so you should try just in case), the next two are colleges that you think are at your level and so you have a good chance of getting in, and the final two are colleges that you are less impressed with, but you think you'd easily be accepted. The reasoning is, if you get accepted to a college that is a reach for you, you'll likely go there. Otherwise, you'll go to one of the colleges in your range assuming you got accepted there. If not, you'll consider going to one of the last two colleges, since there's a pretty big chance you will have been accepted to those.
Note also that "a reach" can be used in other circumstances that don't necessarily mean literally reaching for something: for instance, a hockey team could be said to be "making a reach for gold".