Respected Balbir singh ji,
I had similar doubt as you have.However, One has to keep in mind that Gurbani is always correct,our understanding is,probably ,poor.It is true for all of us.We, sometimes, get stuck up on account of limtations that our intellect is not ready to accept the meaning as it appears on the face.
'Shabd' in two context.
1.The above word can be read/used in two contexts.One is that 'shabad' is the eternal sound or the sound current.It is advocated by of some spritual masters that ultimately the objective is to get hold of this in essence 'shabad'.
2.The second meaning of the 'shabad' is the 'word' used to mean Gurbani.One can infer any meaning. One is free to use the present word under consideration.However, by using the meaning one should try to reach some logical meaning.If the meaning that we assign to the Gurbani leads to absurdity the same should be avoided.
I should suggest you that you look into the above sentence again.It is as follows:
" ਗੁਰ ਕੈ ਸਬਦਿ ਮਰੈ ਮਨੁ ਮਾਰੇ ਹਉਮੈ ਜਾਇ ਸਮਾਇਆ"
I would look into the above sentence by using some spacing to arrive at the meaning that should be logical.I would read the above lline as follows:
"ਗੁਰ ਕੈ ਸਬਦਿ , ਮਰੈ ਮਨੁ , ਮਾਰੇ ਹਉਮੈ , ਜਾਇ ਸਮਾਇਆ."
I have simply added two commas,Hope there won't be any objection.Even "japu ji sahib' requires this sort of mental placement of the commas..Most of the Gurbani requires this sort reading.
On reading the above there is a clear meaning that thru. Guru's Shabad one kills the mind [ controls the uncontrolled mind].
Guru's shabad kills the egoism.
Thru Guru's shabad one intermingles with the Almighty.
Hope this should suffice.I have made an attempt for myself and am sharing with you only and I may not be correct.
Thx and regards.
[ I would be sorry for all the mistakes.]
hps