TL;DR: Your colleague has added the definite article because his/her version has an implicit verb which requires it. However, your colleague has introduced an inaccuracy and their version is harder to parse. On the whole the original is better.
[The explanation which follows is somewhat long for such a minor difference, but it is a very slight one which we need to dig into the grammar to see]
Metaphorical language
First of all, keep in mind that you are speaking somewhat metaphorically about closures and openings "reaching" or "being at" or "hitting" certain levels. This is what allows you to reference "the level of openings" instead of "the number of openings." It's not wrong; in business/economics contexts it is fully appropriate, quite common, and it adds some nice color to the text (think of e.g. "our sales just hit 4,000!!"). However, keep in mind that strictly speaking it's not "closures" which are at 4,000, it is the number of closures. This means that it is unnatural to say, for example, "sales are 4000."
Parsing your version
Your version has
[openings were] reaching almost the same level as closures in 2014 at just over 6,000.
The phrase "at just over 6,000" may be understood to be as an adverb modifying "reaching" (the object of which is "almost the same level as the closures in 2014"). In this parsing, openings are reaching just over 6,000. It may also be (with an implicit verb "to be") modifying "closures," making the clause mean something closer to "...reaching almost the same level as closures (which were at just over 6,000) in 2014."
Therefore, notice that there is an ambiguity in your versions at to what is "at just over 6,000." Is it the closures or the openings? This is inherent to the grammar of the sentence, though not a mistake, and also not a problem since the two values (or the closures & the openings) are stated to be similar. Note also that while both closures and openings can be "at" 6,000, neither can (idiomatically) "be" 6,000.
Parsing your colleague's version without 'the'
[openings were] reaching nearly the same level as closures, nearly 6,000, in 2014.
This time, let's see what happened if we take the phrase "nearly 6,000" as modifying "closures." If we do so, wwe parse it as "...reaching almost the same level as closures (which were nearly 6,000)." Yet "closures" cannot "be" nearly 6,000 for the reasons stated above: we are speaking metaphorically. Therefore it be taken as modifying "reaching." At the same time however, "nearly 6,000" is placed in commas, which separate it from "reaching" and make parsing it this way (as an adverb) quite difficult. Therefore the reader has to either contend with the slight un-idiomaticity of "closures are nearly 6,000" or make the unintuitive leap to see "nearly 6,000" as modifying "reaching."
With 'the'
Therefore your colleague adds "the." This makes having "nearly 6,000" modifying "closures" possible, for now we can read an implicit "number of" between "the" and "closures." Parsing it this time, we have "...reaching almost the same level as the [number of] closures (which were nearly 6,000)." This makes sense, but it's somewhat demanding of the reader. Further, by placing "nearly 6,000" in commas, it separates it from "reaching", so it is very difficult to read "nearly 6,000" as an adverb as we did before.
In sum, this version makes it so that essentially the only way to parse the sentence is to understand the level of closures to be "nearly 6,000*. This resolves the ambiguity of your version, but it is difficult to parse because the reader must supply a phrase ("number of") to make it fully grammatical. You might think to fix this by adding the phrase in explicitly, but this highlights another problem with your colleague's version: that it disrupts the nice parallel between "openings" and "closings" which is present in yours; this version has "openings" and "the closings." "Openings" and "the number of closings" would be even less nice.
Nearly?
But why has your partner made it say "nearly 6,000"? In the graph both points are clearly over 6,000, so this is an error.
Conclusion
So your colleague has resolved the ambiguity of "what does 6,000 apply to?" Of course, this was not an important ambiguity, since the whole point is the values are close. But the solution is an awkward one, and there is not an easy way to make it more natural. Further, they have introduced an error.
