The interpretation of a sentence in the Qieyun (切韵) preface
There are many errors in the 百度百科's article you gave, so I show my punctuation of the preface first.
The content comes from the 水巷孑蠻's answer.


  1) 昔，開皇初，有儀同劉臻等八人，同詣法言門宿。
  2) 夜永酒闌，論及音韻。
  3) 以今聲調，既自有別，諸家取捨，亦復不同。
  4) 吳楚，則時傷輕淺；燕趙，則多傷重濁；秦隴，則去聲為入；梁益，則平聲似去。
  5) 又，支（章移切）脂（旨夷切）、魚（語居切）虞（遇俱切），共為一韻；先（蘇前切）仙（相然切）、尤（于求切）侯（胡溝切），俱論是切。
  6) 欲廣文路，自可清、濁皆通；若賞知音，即須輕、重有異。
  7) 呂靜《韻集》、夏侯該《韻略》、陽休之《韻略》、周思言《音韻》、李季節《音譜》、杜臺卿《韻略》等，各有乖互。
  8) 江東取韻與河北復殊。
  9) 因論南北是非，古今通塞。
  10) 欲更捃選精切，除削疏緩，蕭、顏多所決定。  
  
  11) 魏著作謂法言曰：「向來論難，疑處悉盡，何不隨口記之？我輩數人定，則定矣！」
  12) 法言即燭下握筆，略記綱紀；博問英辯，殆得精華。
  13) 於是，更涉餘學，兼從薄宦，十數年間，不遑修集。  
  
  14) 今反初服，私訓諸弟子：「凡有文藻，即須明聲韻。」
  15) 屏居山野，交游阻絕，疑惑之所，質問無從。
  16) 亡者，則生死路殊，空懷可作之歎；存者，則貴賤禮隔，以報絕交之旨。
  17) 遂取諸家音韻、古今字書，以前所記者，定之為《切韻》五卷，剖析毫氂，分別黍累。  
  
  18) 何煩泣玉，未得縣金。
  19) 藏之名山，昔怪馬遷之言大；持以蓋醬，今歎楊雄之口吃。
  20) 非是小子專輒，乃述羣賢遺意。
  21) 寧敢施行人世？直欲不出戶庭。  
  
  22) 于時，歲次辛酉，大隋仁壽元年。


Here, 陸法言 stated the reason that he wrote this book.
1) and 2) were the story why the subject was raised. (The eight guests actually visited 陸法言's father, 陸爽.)
3) to 8) were the phenomenons they found.
9) and 10) were what they did after finding those phenomenons.
11) and 12) were the reason that 陸法言 started to write. (Because he was the youngest one among them, I think.)
13) was the reason that the book was delayed about 18 years.
14) to 16) were the situations 陸法言 faced after he was fired.
17) was the way he wrote the book.
18) and 21) were what he felt after finishing the writing.
22) was the date.



The punctuation of the sentence in question should be as follows.


  支脂、魚虞，共為一韻；先仙、尤侯，俱論是切。


This sentence has nothing to do with the previous and the next ones.
Its meaning is as follows.  

支、脂混用一韻。
魚、虞混用一韻。
先、仙混用一韻。
尤、侯混用一韻。  

There are four groups here, not just two.
俱論是切 is the same as 共為一韻 in this case.
They all mean that the rhyme of each pair was mixed up.
The author used different expressions to avoid the repetition.
This 切 means the 下字 of the 反切, i.e. the 韻母.
This sentence states that the rhyme of each pair should be different in their opinions, but it was mixed up in some areas or books.
俱, 論, 是 and 切 are equivalent to 共, 為, 一 and 韻 respectively.
論為 means 被當成, that is to be treated as.
是 here means 此 or 這個, i.e. this.
Since it is singular, 是 means 此一 or 這一個, which implies 同一個, the same one.



There are many errors in the 百度百科's interpretation you gave.
北京 首都师范大学 文学院 冯蒸教授 has a much better interpretation, 《切韻˙序》今譯與新注.
He is an expert in this field.

Although he is a native Chinese in China, this article was still written in Traditional Chinese.
It is a copyrighted article.
Open 大江東去, and search 切韻序今譯與新注.